CAs this principle is not universally understood, it may be well to remark that it results immediately from Kirchoff’s law of the proportionality between the radiating and absorbing powers of all bodies for light of each separate wave-length. When a body, even if gaseous in form, is of such great size and density that light of no color can pass entirely through it, then the consequent absorption by the body of light of all colors shows that throughout the region where the absorption occurs there must be an emission of light of these same colors. Thus light from all parts of the spectrum will be emitted by the entire mass.
CAs this principle is not universally understood, it may be well to remark that it results immediately from Kirchoff’s law of the proportionality between the radiating and absorbing powers of all bodies for light of each separate wave-length. When a body, even if gaseous in form, is of such great size and density that light of no color can pass entirely through it, then the consequent absorption by the body of light of all colors shows that throughout the region where the absorption occurs there must be an emission of light of these same colors. Thus light from all parts of the spectrum will be emitted by the entire mass.
A gaseous mass, so large as to be opaque, would, if it were of the same temperature inside and out, give a continuous spectrum, without any dark lines. But the laws of temperature in such a mass show that it will be cooler at the surface than in the interior. This cooler envelope will absorb the rays emanating from the interior as in the case when the latter is solid. We conclude, therefore, that the fact that the great majority of stars show a spectrum like that of the sun, namely, a continuous one crossed by dark lines, does not throw any light on the question whether the matter composing the body of the star is in a solid, liquid or gaseous state. The fact is that the most plausible theories of the constitution of the sun lead to the conclusion that its interior mass is really gaseous. Only the photosphere may be to a greater or less extent solid or liquid. The dark lines that we see in the solar spectrum are produced by the absorption of a comparatively thin and cool layer of gas resting upon the photosphere. Analogy as well as the general similarity of the spectra lead us to believe that the constitution of most of the stars is similar to that of the sun.
When the spectra of thousands of stars were recorded for study, such a variety was found that some system of classification was necessary. The commencement of such a system was made by Secchi in 1863. It was based on the observed relation between the color of a star and the general character of its spectrum.
Arranging the stars in a regular series, from blue in tint through white to red, it was found that the number and character of the spectral lines varied in a corresponding way. The blue stars, like Sirius, Vega and α Aquilæ, though they had the F lines strong, as well as the two violet lines H, had otherwise only extremely fine lines. On the other hand, the red stars, like α Orionis and α Scorpii, show spectra with several broad bands. Secchi was thus led to recognize three types of spectra, as follows:
The first type is that of the white or slightly blue stars, like Sirius, Vega, Altair, Rigel, etc. The typical spectrum of these stars shows all seven spectral colors, interrupted by four strong, dark lines, one in thered, one in the bluish green, and the two others in the violet. All four of these lines belong to hydrogen. Their marked peculiarity is their breadth, which tends to show that the absorbing layer is of considerable thickness or is subjected to a great pressure. Besides these broad rays, fine metallic rays are found in the brighter stars of this type. Secchi considers that this is the most numerous type of all, half the stars which he studied belonging to it.
Fig. 1. Spectrum of Sirius.
Fig. 1. Spectrum of Sirius.
Fig. 2. Spectra ofαAurigæ and Sun.
Fig. 2. Spectra ofαAurigæ and Sun.
Fig. 3. Spectra ofαBootis andβGeminorum.
Fig. 3. Spectra ofαBootis andβGeminorum.
The second type is that of the somewhat yellow stars, like Capella, Pollux, Arcturus, Procyon, etc. The most striking feature of the spectrum of these stars is its resemblance to that of our sun. Like the latter, it is crossed by very fine and close black rays. It would seem that the more the star inclines toward red, the broader these rays become and the easier it is to distinguish them. We give a figure showing the remarkable agreement between the spectrum of Capella, which may be taken as an example of the type, and that of the sun.
The spectra of the third type, belonging mostly to the red stars, are composed of a double system of nebulous bands and dark lines. The latter are fundamentally the same as in the second type, the broad nebulous bands being an addition to the spectrum. α Herculis may be taken as an example of this type.
Fig. 7. Spectrum with both Bright and Dark Lines.
Fig. 7. Spectrum with both Bright and Dark Lines.
It is to be remarked that, in these progressive types, the brilliancy of the more refrangible end of the spectrum continually diminishes relatively to that of the red end. To this is due the gradations of color in the stars.
To these three types Secchi subsequently added a fourth, given by comparatively few stars of a deep red color. The spectra of this class consist principally of three bright bands, which are separated by dark intervals. The brightest is in the green; a very faint one is in the blue; the third is in the yellow and red, and is divided up into a number of others.
To these types a fifth was subsequently added by Wolf and Rayet, of the Paris Observatory. The spectra of this class show a singular mixture of bright lines and dark bands, as if three different spectra were combined, one continuous, one an absorption spectrum, and one an emission spectrum from glowing gas. Less than a hundred stars of this type have been discovered. A very remarkable peculiarity, which weshall discuss hereafter, is that they are nearly all situated very near the central line of the Milky Way.
Fig. 4. Spectra ofαCygni andαTauri.
Fig. 4. Spectra ofαCygni andαTauri.
Fig. 5. Spectrum ofαOrionis.
Fig. 5. Spectrum ofαOrionis.
Fig. 6. Spectrum ofγCassiopeiæ
Fig. 6. Spectrum ofγCassiopeiæ
Vogel proposed a modification of Secchi’s classification, by subdividing each of his three types into two or three others, and including the Wolf-Rayet stars under the second type. His definitions are as follows:
Type I is distinguished by the intensity of the light in the more refrangible end of the spectrum, the blue and violet. The type may be divided into three subdivisions, designateda,bandc:
In Iathe metallic lines are very faint, while the hydrogen lines are distinguished by their breadth and strength.
In Ibthe hydrogen lines are wanting.
In Icthe lines of hydrogen and helium both show as bright lines. Stars showing this spectrum are now known as helium stars.
According to Vogel, the spectra of type II are distinguished by having the metallic lines well-marked and the more refrangible end of the spectrum much fainter than in the case of type I. He recognizes two subdivisions:
In IIathe metallic lines are very numerous, especially in the yellow and green. The hydrogen lines are strong, but not so striking as in Ia.
In IIbare found dark lines, bright lines and faint bands. In this subdivision he includes the Wolf-Rayet stars, more generally classified as of the fifth type.
The distinguishing mark of the third type is that, besides dark lines, there are numerous dark bands in all parts of the spectrum, and the more refrangible end of the latter is almost wanting. There are two subdivisions of this type:
In IIIathe broad bands nearest the violet end are sharp, dark and well-defined, while those near the red end are ill-defined and faint. In IIIbthe bands near the red end are sharp and well-defined; those toward the violet faint and ill-defined. The character of the bands is therefore the reverse of that in subdivisiona.
This classification of Vogel is still generally followed in Germany and elsewhere. It is found, however, that there are star spectra of types intermediate to all these defined. Moreover, in each type the individual differences are so considerable that there is no well-defined limit to the number of classes that may be recognized. At the Harvard Observatory a classification quite different from that of Vogel has been used, but it is too detailed for presentation here. The stars of type II are frequently termed Capellan stars, or Solar stars. Certain stars of type I are termed Orion stars, owing to the number of stars of the type found in that constellation. The stars which show the lines of helium are known as helium stars. We mention these designations because they frequently occur in literature. It would, however, be outside the object of the present work to describe all these classifications in detail. We thereforeconfine ourselves to a few illustrations of spectra of the familiar types described by Secchi and Vogel.
There are many star spectra which cannot be included in any of the classes we have described. Up to the present time these are generally described as stars of peculiar spectra.
As the present chapter is confined to the more general side of the subject, we shall not attempt any description of special spectra. These, especially the peculiar spectra of the nebulæ, of new stars, of variable stars, etc., will be referred to, so far as necessary, in the chapters relating to those objects.
The most interesting conclusion drawn from observations with the spectroscope is that the stars are composed, in the main, of elements similar to those found in our sun. As the latter contains most of the elements found on the earth and few or none not found there, we may say that earth and stars seem to be all made out of like matter. It is, however, not yet easy to say that no elements unknown on the earth exist in the heavens. It would scarcely be safe to assume that, because the line of some terrestrial substance is found in the spectrum of a star, it is produced by that substance. It is quite possible that an unknown substance might show a line in appreciably the same position as that of some substance known to us. The evidence becomes conclusive only in the case of those elements of which the spectral lines are so numerous that when they all coincide with lines given by a star, there can be no doubt of the identity.
We may assume that the stars are all in motion. It is true that only a comparatively small number of stars have been actually seen to be in motion; but as some motion exists in nearly every case where observations would permit of its being determined, we may assume the rule to be universal. Moreover, if a star were at rest at one time it would be set in motion by the attraction of other stars.
Statements of the motion from different points of view illustrate in a striking way the vast distance of the stars and the power of modern telescopic research. If Hipparchus or Ptolemy should rise from their sleep of 2,000 years—nay, if the earliest priests of Babylon should come to life again and view the heavens, they would not perceive any change to have taken place in the relative positions of the stars. The general configurations of the constellations would be exactly that to which they were accustomed. Had they been very exact observers they might notice a slight difference in the position of Arcturus; but as a general rule the unchangeability would have been manifest.
In dealing with the subject, the astronomer commonly expresses themotion in angular measurement as so many seconds per year or per century. The keenest eye would not, without telescopic aid, be able to distinguish between two stars whose apparent distance is less than 2′ or 120″ of arc. The pair of stars known as (ε) Lyræ are 3′ apart; yet, to ordinary vision they appear simply as a single star. To appreciate what 1″ of arc means we must conceive that the distance between these two stars is divided by 200. Yet this minute space is easily distinguished and accurately measured by the aid of a telescope of ordinary power.
On the other hand, if we measure the motions by terrestrial standards they are swift indeed. Arcturus has been moving ever since the time of Job at the rate of probably more than 200 miles per second—possibly 300 miles. Generally, however, the motion is much smaller, ranging from an imperceptible quantity up to 5, 10 or 20 miles a second. Slow as the angular motion is, our telescopic power is such that the motion in the course of a very few years (with Arcturus the motion in a few days) can be detected. As accurate determinations of positions of the stars have been made only during a century and a half, no motions can be positively determined except those which would become evident to telescopic vision in that period. Only about 3,000 stars have been accurately observed so long as this. In the large majority of cases the interval of observation is so short or the motion so slow that nothing can be asserted respecting the law of the motion.
The great mass of stars seem to move only a few seconds per century, but there are some whose motions are exceptionally rapid. The general rule is that the brighter stars have the largest proper motions. This is what we should expect, because in the general average they are nearer to us, and therefore their motion will subtend the greatest angle to the eye. But this rule is only one of majorities. As a matter of fact, the stars of largest proper motion happen to be low in the scale of magnitude. It happens thus because the number of stars of smaller magnitudes is so much greater than that of the brighter ones that the very small proportion of large proper motions which they offer over-balances those of the brighter stars.
The discovery of the star of greatest known proper motion was made by Kapteyn, of Groningen, in 1897, coöperating with Gill and Innes, of the Cape Observatory. While examining the photographs of the stars made at this institution, Kapteyn was surprised to notice the impression of a star of the eighth magnitude which at first could not be found in any catalogue. But on comparing different star lists and different photographs it soon became evident that the star had been previously seen or photographed, but always in slightly varying positions. An examination of the observed positions at various times showed that the star had a more rapid proper motion than any other yet known. Yet, greatthough this motion is, it would require nearly 150,000 years for the star to make a complete circuit of the heavens if it moved round the sun uniformly at its present rate.
The fact that the stars move suggests a very natural analogy to the solar system. In the latter a number of planets revolve round the sun as their center, each planet continually describing the same orbit, while the various planets have different velocities. Around several of the planets revolve one or more satellites. Were civilized men ephemeral, observing the planets and satellites only for a few minutes, these bodies would be described as having proper motions of their own, as we find the stars to have. May it not then be that the stars also form a system; that each star is moving in a fixed orbit performing a revolution around some far-distant center in a period which may be hundreds of thousands or hundreds of millions of years? May it not be that there are systems of stars in which each star revolves around a center of its own while all these systems are in revolution around a single center?
This thought has been entertained by more than one contemplative astronomer. Lambert’s magnificent conception of system upon system will be described hereafter. Mädler thought that he had obtained evidence of the revolution of the stars around Alcyone, the brightest of the Pleiades, as a center. But, as the proper motions of the stars are more carefully studied and their motion and direction more exactly ascertained, it becomes very clear that when considered on a large scale these conceptions are never realized in the actual universe as a whole. But there are isolated cases of systems of stars which are shown to be in some way connected by their having a common proper motion. We shall mention some of the more notable cases.
The Pleiades are found to move together with such exactness that up to the present time no difference in their proper motions has been detected. This is true not only of the six stars which we readily see with the naked eye, but of a much larger number of fainter ones made known by the telescope. It is an interesting fact, however, that a few stars apparently within the group do not partake of this motion, from which it may be inferred that they do not belong to the system. But there must be some motion among themselves, else the stars would ultimately fall together by their mutual attraction. The amount and nature of this motion cannot, however, be ascertained except by centuries of observation.
Another example of the same sort is seen in five out of the seven stars of Ursæ Major, or The Dipper. The stars are those lettered β, γ, δ, ε and θ. All five have a proper motion in R. A. of nearly 8″ per century, while in declination the movements are sometimes positive and sometimes negative; that is to say, some of the stars are apparently lessening their distance from the pole, whileothers are increasing it. But when we project the motions on a map we find that the actual direction is very nearly the same for all five stars, and the reason why some move slightly to the north and others slightly to the south is due to the divergence of the circles of right ascension. It is worthy of remark that the community of motion is also shown by spectroscopic observations of the radial motions described below.
The five stars in question are all of the second magnitude except δ, which is of the third. It is a curious fact that no fainter stars than these five have been found to belong to the system.
From a study of these motions Höffler has concluded that the five stars lie nearly in the same plane and have an equal motion in one and the same direction. From this hypothesis he has attempted to make a determination of their relative and actual distances. The result reached in this way cannot yet, however, be regarded as conclusive.
There are three stars in Cassiopeia, β, η and μ each having a large proper motion in so nearly the same direction that it is difficult to avoid at least a suspicion of some relation between them. The angular motions are, however, so far from equal that we cannot regard the relation as established.
In the constellation Taurus, between Aldebaran and the Pleiades, most of the stars which have been accurately determined seem to have a common motion. But these motions are not yet so well ascertained that we can base anything definite upon them. They show a phenomenon which Proctor very aptly designated as star-drift.
The systems we have just described comprise stars situated so far apart that, but for their common motion, we should not have suspected any relation between them. The community of origin which their connection indicates is of great interest and importance, but the question belongs to a later chapter.
No achievement of modern science is more remarkable than the measurement of the velocity with which stars are moving to or from us. This is effected by means of the spectroscope through a comparison of the position of the spectral lines produced by the absorption of any substance in the atmosphere of the star with the corresponding lines produced by the same substance on the earth. The principle on which the method depends may be illustrated by the analogous case of sound. It is a familiar fact that if we stand alongside a railway while a locomotive is passing us at full speed, and at the same time blowing a whistle, the pitch of the note which we hear from the whistle is higher as the engine is approaching than after it passes. The reason is that the pitch of a sound depends upon the number of sound beats per second.
A B X* . . . . . . .* . . . . . . .
A B X* . . . . . . .* . . . . . . .
Now, we may consider the waves which form light when they strike our apparatus as beats in the ethereal medium which follow each other with extraordinary rapidity, millions of millions in a second, moving forward with a definite velocity of more than 186,000 miles a second. Each spectral line produced by a chemical element shows that that element, when incandescent, beats the ether a certain number of times in a second. These beats are transmitted as waves. Since the velocity is the same whether the number of beats per second is less or greater, it follows that, if the body is in motion in the direction in which it emits the light, the beats will be closer together than if it is at rest; if moving away they will be further apart. The fundamental fact on which this result depends is that the velocity of the light-beat through the ether is independent of the motion of the body causing the beat. To show the result, let A be a luminous body at rest; let the seven dots to the right of A be the crests of seven waves or beats, the first of which, at the end of a certain time, has reached X. The wave-length will then be one seventh the distance A X. Now, suppose A in motion toward X with such speed that, when the first beat has reached X, A has reached the point B. Then the seven beats made by A while the first beat is traveling from A to X, and A traveling from A to B, will be crowded into the space B X, so that each wave will be one seventh shorter than before. In other words, the wave-lengths of the light emitted by any substance will be less or greater than their normal length, according to the motion of the substance in the direction in which its light is transmitted, or in the opposite direction.
The position of a ray in the spectrum depends solely on the wave-length of the light. It follows that the rays produced by any substance will be displaced toward the blue or red end of the spectrum, according as the body emitting or absorbing the rays is moving towards or from us. This method of determining the motions of stars to or from us, or their velocity in the line of sight from us to the star, was first put into practice by Mr.—now Sir William—Huggins, of London. The method has since been perfected by photographing the spectrum of a star, or other heavenly body, side by side with that of a terrestrial substance, rendered incandescent in the tube of a telescope. The rays of this substance pass through the same spectroscope as those from the star, so that, if the wave-lengths of the lines produced by the substance were the same as those found in the star spectrum, the two lines would correspond in position. The minute difference found on the photographic plate is the measure of the velocity of the star in the line of sight.
It will be seen that the conclusion depends on the hypothesis thatthe position of any ray produced by a substance is affected by no cause but the motion of the substance. How and when this hypothesis may fail is a very important question. It is found, for example, that the position of a spectral ray may be altered by compressing the gas emitting or absorbing the ray, and it may be inquired whether the results for motion in the line of sight may not be vitiated by the absorbing atmosphere of the star being under heavy pressure, thus displacing the absorption line.
To this it may be replied that, in any case, the outer layers of the atmosphere, through which the light must last pass, are not under pressure. How far inner portions may produce an absorption spectrum we cannot discuss at present, but it does not seem likely that serious errors are thus introduced in many cases.
These measures require apparatus and manipulation of extraordinary delicacy, in order to avoid every possible source of error. The displacement of the lines produced by motion is in fact so minute that great skill is required to make it evident, unless in exceptional cases. The Mills spectrograph of the Lick Observatory in the hands of Professor Campbell has, notwithstanding these difficulties, yielded results of extraordinary precision. Quite a number of investigators at some leading observatories of Europe and America are pursuing the work of determining these motions. The determinations have almost necessarily been limited to the brighter stars, because, owing to the light of the star being spread over so broad a space in the spectrum, instead of being concentrated on a point, a far longer exposure is necessary to photograph the spectrum of a star than to photograph the star itself. The larger the telescope the fainter the star whose spectrum can be photographed. Vogel, of Potsdam, who has made the most systematic sets of these measures that have yet appeared, included few stars fainter than the second magnitude. With the largest telescopes the spectro of stars down to about the fifth magnitude may be photographed; beyond this it is extremely difficult to go. The limit will probably be reached by the spectrograph of the Yerkes Observatory, which is now being put into operation by Professors Hale and Frost.
When a star is found to be seemingly in motion, as described in the last section, we may ascribe the phenomenon to a motion either of the star itself or of the observer. In fact no motion can be determined or defined except by reference to some body supposed to be at rest. In the case of any one star, we may equally well suppose the star to be at rest and the observer in motion, or the contrary. Or we may suppose both to have such motions that the difference of the two shall represent theapparent movement of the star. Hence our actual result in the case of each separate star is a relation between the motion of the star and the motion of the sun.
I say the motion of the sun and not of the earth, because although the observer is actually on the earth, yet the latter never leaves the neighborhood of the sun, and, as a matter of fact, the ultimate result in the long run must be a motion relative to the sun itself as if we made our observations from that body. The question then arises whether there is any criterion for determining how much of the apparent motion of any given star should be attributed to the star itself and how much to a motion of the sun in the opposite direction.
If we should find that the stars, in consequence of their proper motions, all appeared to move in the same direction, we would naturally assume that they were at rest and the sun in motion. A conclusion of this sort was first reached by Herschel, who observed that among the stars having notable proper motions there was a general tendency to move from the direction of the constellation Hercules, which is in the northern hemisphere, towards the opposite constellation Argo, in the southern hemisphere.
Acting on this suggestion, subsequent astronomers have adopted the practice of considering the general average of all the stars, or a position which we may regard as their common center of gravity, to be at rest, and then determining the motion of the sun with respect to this center. Here we encounter the difficulty that we cannot make any absolute determination of the position of any such center. The latter will vary according to what particular stars we are able to include in our estimate. What we can do is to take all the stars which appear to have a proper motion, and determine the general direction of that motion. This gives us a certain point in the heavens toward which the solar system is traveling, and which is now called thesolar apex, or the apex of the solar way.
The apparent motion of the stars due to this motion of the solar system is now called theirparallactic motion, to distinguish it from the actual motion of the star itself.
The interest which attaches to the determination of the solar apex has led a great number of investigators to attempt it. Owing to the rather indefinite character of the material of investigation, the uncertainty of the proper motions, and the additions constantly made to the number of stars which are available for the purpose in view, different investigators have reached different results. Until quite recently, the general conclusion was that the solar apex was situated somewhere in the constellation Hercules. But the general trend of recent research has been to place it in or near the adjoining constellation Lyra. Thischange has arisen mainly from including a larger number of stars, whose motions were determined with greater accuracy.
Former investigators based their conclusions entirely on stars having considerable proper motions, these being, in general, the nearer to us. The fact is, however, that it is better to include stars having a small proper motion, because the advantage of their great number more than counterbalances the disadvantage of their distance.
The conclusions reached by some recent investigators of the position of the solar apex will now be given. We call A the right ascension of the apex; D its declination.
Prof. Lewis Boss, from 273 stars of large proper motion found
A = 283°.3; D = 44°.1.
If he excluded the motions of 26 stars which exceeded 40″ per century the result was
A = 288°.7; D = 51°.5.
A comparison of these numbers shows how much the result depends on the special stars selected. By leaving out 26 stars the apex is changed by 5° in R. A., and 7° in declination.
It is to be remarked that the stars used by Boss are all contained in a belt four degrees wide, extending from 1° to 5° north of the equator.
Dr. Oscar Stumpe, of Berlin, made a list of 996 stars having proper motions between 16″ and 128″ per century. He divided them into three groups, the first including those between 16″ and 32″; the second between 32″ and 64″; the third between 64″ and 128″. The number of stars in each group and the position of the apex derived from them are as follows:
Porter, of Cincinnati, made a determination from a yet larger list of stars with results of the same general character.
These determinations have the advantage that the stars are scattered over the entire heavens, the southern as well as the northern ones. The difference of more than 10° between the position derived from stars with the largest proper motions, and from the other stars, is remarkable.
The present writer, in a determination of the precessional motion, incidentally determined the solar motion from 2,527 stars contained in Bradley’s Catalogue which had small proper motions, and from about 600 more having larger proper motions. Of the latter the declinations only were used. The results were:
From all these results it would seem that the most likely apex of the solar motion is toward the point in
This point is situated in the constellation Lyra, about 2° from the first magnitude star Vega. The uncertainty of the result is more than this difference, four or five degrees at least. We may therefore state the conclusion in this form:
The apex of the solar motion is in the general direction of the constellation Lyra, and probably very near the star Vega, the brightest of that constellation.
It must be admitted that the wide difference between the position of the apex from large and from small proper motions, as found by Porter, Boss and Stumpe, require explanation. Since the apparent motions of the stars are less the greater their distance, these results, if accepted as real, would lead to the conclusion that the position of the solar apex derived from stars near to us was much further south than when derived from more distant stars. This again would indicate that our sun is one of a cluster or group of stars, having, in the general average, a different proper motion from the more distant stars. But this conclusion is not to be accepted as real until the subject has been more exhaustively investigated. The result may depend on the selection of the stars; and there is, as yet, no general agreement among investigators as to the best way of making the determination.
The next question which arises is that of the velocity of the solar motion. The data for this determination are more meagre and doubtful than those for the direction of the motion. The most obvious and direct method is to determine the parallactic motion of the stars of known parallax. Regarding any star 90° from the apex of the solar motion as in a state of absolute rest, we have the obvious rule that the quotient of its parallactic motion during any period, say a century, divided by its parallax, gives the solar motion during that period, in units of the earth’s distance from the sun. In fact, by a motion of the sun through one such unit, the star would have an apparent motion in the opposite direction equal to its annual parallax. If the star were not 90° from the apex we can easily reduce its observed parallactic motion by dividing it by the sign of its actual distance from the apex.
Since every star has, presumably, a proper motion of its own, we can draw no conclusion from the apparent motion of any one star, owing to the impossibility of distinguishing its actual from its parallactic motion. We should, therefore, base our conclusion on the mean result from a great number of stars, whose average position or center of mass we might assume to be at rest. Here we meet the difficulty that there areonly about 60 stars whose parallaxes can be said to be determined; and one-half of these are too near the apex, or have too small a parallax, to permit of any conclusion being drawn from them.
A second method is based on the spectroscopic measures of the motion of stars in the line of sight, or the line from the earth to the star. A star at rest in the direction of the solar apex would be apparently moving toward us with a velocity equal to that of the solar motion. Assuming the center of mass of all the stars observed to be at rest, we should get the solar motion from the mean of all.
Thus far, however, there are only about 50 stars whose motions in the line of sight have been used for the determination, so that the data are yet more meagre than in the case of the proper motions. From them, however, using a statistical method Kapteyn has derived results which seem to show that the actual velocity of the solar system through space is about 16 kilometres, or 10 statute miles, per second.
Thefacts and considerations we have passed in review fairly indicate the physiological and psychological preëminence of red among the colors of the spectrum to which we are sensitive. What is the cause of that preëminence?
It seems to me that two orders of causes have coöperated to produce this predominant influence, one physical and depending on the special effects of the long-waved portion of the spectrum on living matter, the other psychological and resulting from the special environmental influences to which man, and to some extent even the higher animals generally, have been subjected. It is possible that these two influences blend together and cannot at any point be disentangled; it is possible that acquired aptitude may be inherited or that what seem to be acquired aptitudes are really perpetuated congenital variations; but on the whole the two influences are so distinct that we may deal with them separately.
On the physical side the influence of the red rays, although there is much evidence showing that it may be traced throughout the whole of organic nature, is certainly most strongly and convincingly exhibited on plants. The characteristic greenness of vegetation alone bears witness to this fact. The red rays are life to the chlorophyll-bearing plant, the violet rays are death. A meadow, it has been justly said, is a vast field of tongues of fire greedily licking up the red rays and vomiting forth the poisonous bile of blue and yellow. An experiment of Flammarion’s has beautifully shown the widely different reaction of plants to the red and violet rays. At the climatological station at Juvisy he constructed four greenhouses—one of ordinary transparent glass, another of red glass, another of green, the fourth of dark blue. The glass was monochromatic, as carefully tested by the spectroscope, and dark blue was used instead of violet because it was impossible to obtain a perfect violet glass. These were all placed under uniform meteorological and other conditions, and from certain plants such as the sensitive plant, previously sown on the same day in the same soil, eight of each kind were selected, all measuring 27 millimetres, and placed by two and two in the four greenhouses on the 4th of July. On the 15th of August there were notable differences in height, color and sensitiveness, and these differences continued to become marked; photographs of theplants on the 4th of October showed that while those under blue glass had made no progress, those under red glass had attained extraordinary development, red light acting like a manure. While those under blue glass became insensitive, under red glass the sensitive plants had become excessively sensitive to the least breath. They also flowered, those under transparent glass being vigorous and showing buds, but not flowering. The foliage under red glass was very light, under blue darkest. Similar but less marked effects were found in the case of geraniums, strawberries, etc. The strawberries under blue glass were no more advanced in October than in May; though not growing old their life was little more than a sleep. It appears, however, that the stimulating influence of red light fails to influence favorably the ripening of fruit. Zacharewiez, professor of agriculture at Vaucluse, has found that red, or rather orange, produces the greatest amount of vegetation, while as regards fruit, the finest and earliest was grown under clear glass, violet glass, indeed, causing the amount of fruit to increase but at the expense of the quality.
Moreover, the lowest as well as the highest plants participated in this response to the red rays, and in even a more marked degree, for they perish altogether under the influence of the violet rays. Marshall Ward and others have shown that the blue, violet and ultra-violet rays, but no others, are deleterious to bacteria. Finsen has successfully made use of this fact in the treatment of bacterial skin diseases. Reynolds Green has shown that while the ultra-violet rays have a destructive influence on diastase, the red rays have a powerfully stimulating effect, increasing diastase and converting zymogen into diastase.
While the influence of the red rays on the plant is thus so enormous and easily demonstrated, the physical effects of red on animals seem to be even opposite in character, although results of experiments are somewhat contradictory. Béclard found that the larvæ of the flesh fly raised under violet glass were three fourths larger than those raised under green glass; the order was violet, blue, red, yellow, white, green. In the case of tadpoles, Yung found that violet or blue was especially favorable to the growth of frogs; he also found that fish hatch most rapidly under violet light. Thus the influence that is practically death to plants is that most favorable to life in animals. Both effects, however, as Davenport truly remarks in his ‘Experimental Morphology,’ when summing up the results of investigations, are due to the same chemical metabolic changes, but while plants succumb to the influence of the violet rays, animals, being more highly organized, are able to take advantage of them and flourish.
At the same time the influence of violet rays on animal tissue is by no means invariably beneficial; they are often too powerful a stimulant. That the violet rays have an influence on the human skin which in thefirst place, at all events, is destructive and harmful in a high degree, is now clearly established by the observations and experiments of Charcot, Unna, Hammer, Bowles and others, while Finsen has made an important advance in the treatment of disease based on this fact. The conditions called ‘sun-burn,’ ‘snow-burn,’ ‘snow-blindness,’ for instance, which may affect even travelers on snow-fields and Arctic explorers, are now known to be wholly due to the violet and not to the red rays. Unna’s device of wearing a yellow veil, and Bowles’s plan of painting the skin brown, thus shutting off the violet rays, suffice to prevent sun-burn. The same effect is also obtained by nature, which under the stress of sunlight, and largely through the irritation of the violet rays themselves, weaves a pigmentary veil of yellow and brown on the skin, which thus protects from the further injurious influence of the violet rays and renders the sunlight a source of less alloyed joy and health.
That the presence of the red rays, or at all events the exclusion of the violet, is of great benefit in many skin diseases seems to be now beyond doubt. This has been shown by Finsen in his treatment of smallpox in red rooms; it appears that it was also known in the Middle Ages as well as in Japan, Tonquin and Roumania, red bed-covers, curtains or carpets being used to obtain the effect. Under the treatment by red light not only is the skin enabled to heal healthfully without scarring, but the whole course of the disease is beneficially affected and abbreviated, the fever is diminished and also the risk of complications. Another physician has discovered that a similar beneficial effect is produced by red light in measles. A child with a severe attack of measles was put into a room with red blinds and a photographic lamp. The rash speedily disappeared and the fever subsided, the child complaining only of the absence of light; the blinds were consequently removed, and the fever, rash and prostration returned, to disappear again when the blinds were resumed.
Whether red light, or the exclusion of violet, exerts a beneficial influence on the hæmoglobin of the blood and on metabolism generally has not been distinctly proved, but it seems to me to be indicated by such experiments as those of Marti published a few years ago in theAtti dei Lincei. This investigator found that while feeble irritation of the skin promotes the formation of blood corpuscles, strong irritation diminishes the blood corpuscles and also the hæmoglobin; at the same time he found that darkness also diminishes the number of red corpuscles, while continued exposure to intense light (even at night the electric light, which, however, is rich in violet rays) favors increased formation of red corpuscles, and in some degree of hæmoglobin. Finsen has shown that inflammation of the skin caused by chemical or violet light leads to contraction of the red corpuscles.
This brings us to the consideration of the influence of the red rayson the nervous system. From time to time experiments have been made as to the influence of various colored lights, chiefly on the insane, as first suggested by Father Secchi in 1895. Even yet, however, the specific mental influences of the various colors are not quite clear. It has been found by some that the red rays are far more soothing and comfortable, less irritating, than the total rays of uncolored light, and Garbini found that angry infants were soothed by the light through red glass, only slightly by that through green and not at all by other colored light. On the other hand, it is stated that a well-known dry plate manufacturer at Lyons was obliged to substitute green-colored glass in the windows of his large room for the usual red because the work people sang and gesticulated all day and the men made love to the women, while under the influence of green glass (which also allows yellow rays to pass) they became quiet and silent and seemed less fatigued when they left off work. We need not attach much value to these statements, but in this connection it is interesting to refer to the results obtained some years ago by Féré and recorded in his ‘Sensation et Mouvement.’ Experimenting on normal subjects as well as on nervous subjects, who were found more sensitive, with colored light passed through glass or sheets of gelatine, he found notable differences in muscular power, measured by the dynamometer, and in the circulation as measured by plethysmographic tracings of the forearm under the influence of different colors. He found in this manner with one subject whose normal muscular power was represented by 23 that blue light increased his power to 24, green to 28, yellow to 30, orange to 35 and red to 42. The dynamogenic powers of the different colors were thus found to rank in the spectral order, red representing the climax of energy, or, as Féré puts it, “the intensity of the visual sensation varies as the vibrations.” Féré found that colors need not be perceived in order to show their influence, thus proving the purely physical nature of that influence, for in a subject who was unable to see colors with one eye, the color stimulus had the same dynamogenic effect whether applied to the seeing or the defective eye. Increase of volume of blood in the limbs, measured by the plethysmograph, so far as we can rely on Féré’s experiments, ran parallel with the influence on muscular power, culminating with red, so that no metaphor is involved, Féré remarks, when we speak of red as a ‘warm’ color. On the insane the results attained by the use of colored glass do not seem to be quite coherent. Some of the earlier observers described the beneficial effects of blue glass in soothing maniacs. Pritchard Davies, however, was not able to find that red light had any beneficial effect, though on some cases blue had, while Roffegean found that, in the case of a somber and taciturn maniac who could rarely be persuaded to eat, three hours in a red-lighted room produced a markedly beneficial effect, and a man withdelusions of persecution became quite rational and was even in a condition to be sent home after a few days in the same room. He also found that a violent maniac wearing a strait jacket, after a few hours in a room with blue glass windows became quite calm and gave no further trouble. Osburne has found, after many years’ experience, that in the absence of structural disease violet light (for from three to six hours) is most useful in the treatment of excitement, sleeplessness and acute mania; red he has found of some benefit, though to a much less degree in such cases (it must be remarked that violet light as usually applied is not free from red), while he has not found any color with which he has tried experiments (red, orange or violet) of benefit in melancholia. The significance of these facts is not altogether clear; the influence, as Pritchard Davies concluded, seems to be largely moral, though it may be that the colors of long wave-length are tonic and those of short wave-length sedative.
So far I have been chiefly concerned to point out that the immense emotional impressiveness of red has a basis in physical laws, being by no means altogether a matter of environmental associations. It is true that the two groups of influences overlap, and that we can not always distinguish them. We can not be sure that the greater sensitiveness to the red rays may not have been emphasized in the organism, not necessarily as the result of inherited acquirement, but probably as the perpetuation of a variation of sensibility, found beneficial in an environment where red was liable to be especially associated with objects that were to be avoided as terrible or sought as useful. In this way the physical and environmental factors would run in a circle.
We have to bear this consideration in mind when we take into account the susceptibilities of animals, especially of the higher animals, to red. The color sense, it is well known, is widely diffused among animals; indeed this fact has been brought forward, especially by Pouchet, to prove that there can have been no color evolution in man; this it can scarcely be said to show, since evolution does not run in a straight line, and it is quite conceivable and even probable that the ancestors of man were less dependent than many lower animals, for the means of living, on a highly developed color sense. Thus a color sense that among some creatures is so highly developed as to include even the ultra-violet rays, was among our own ape-like ancestors either never developed or partially lost.
Graber, in his important investigation into the color sense of animals, showed that of fifty animals studied by him forty showed strong color preferences in their places of abode. In general he found, without being able to explain the fact, that animals which prefer the dark are red lovers, those which prefer the light are blue lovers. The common worm, with head and tail cut off, still preferred red to blue nearly asmuch as when uninjured. (This would seem to indicate the same kind of susceptibility to unaccustomed violet rays which we have already encountered in the phenomenon of sun-burn.) The triton and cochineal, with eyes removed and heads covered with wax, still had delicate sense for color and brightness. The flea infesting the dog had a finer color sense than the bee, while nearly all the animals Graber investigated were more or less sensitive to the ultra-red rays.
Among insects it scarcely appears, nor should we expect that there would be any peculiarly marked predilection or aversion for red. Cockerell and F. W. Anderson, from observations in various parts of the United States, believe that yellow (i. e., the brightest color) is the most attractive to insects, and the former doubts whether insects can distinguish red from yellow. Among the higher animals, and even among fishes and birds, there is not only a color sense, but a highly emotionalized color sense, and red appears to be usually the color that arouses the emotion. There is a proverb, ‘Women and mackerel are caught by red,’ and perch is also said to be caught by red bait. Sparrows appear to be repelled by red; the case is reported of a hen sparrow, kept in captivity for ten years, which though otherwise a fearless bird ‘would on seeing scarlet show painful signs of distress and faint away.’ The lady who records this observation has noted the same repugnance to red, though in a less marked degree, in other sparrows, one of which showed a predilection for blue objects, and she remarks that when feeding outdoor sparrows from the window they flew away when she wore a red jacket, while a blue jacket inspired them with confidence; other birds, she found, except a cockatoo, were unaffected by colors. Red, it is well known, is very obnoxious to turkey cocks, while the fury aroused in various quadrupeds by red was known at a very early period; Seneca referred to it in the case of the bull, the most familiar example; it is seen in buffaloes, sometimes in horses, and also, it is said, in the hippopotamus.
The phenomena of color aversion and color predilection among insects may possibly be in some degree a matter of physical sensibility, varying according to the creature’s tissues, habitat and needs, but as we approach the vertebrates and especially the mammals there can be little doubt that it is mainly a matter of environment and association; in other words, that it is accounted for by the color of food, the color of blood and the color of the chief secondary sexual characters.
Let us, however, confine ourselves to man, and consider what are the chief colored objects that are of most vital concern to the human and most closely allied species.
One of the earliest groups of such objects—some would say the most important group in this connection—is that of ripe fruits. Certainly among the frugivorous apes and among many races of primitiveman, the color of fruits must be a powerful factor in developing a sensibility for red rays, and in associating such sensibility with emotional satisfaction. The color of fruits is most generally red, orange or purple, and since purple is largely made up of red, it is clear that the influence of fruits will almost exclusively bear on the rays of long wave-length. We may reasonably suppose that the search for fruits acted as an important factor in the development of a special sensibility for red.
A later factor in the predilection for the red, orange and yellow rays, though scarcely a factor in their discrimination, lies in the fact that these are the colors of fire. Flame, apart from its beauty, on which certain poets, Shelley especially, have often insisted, is a source of massive physical satisfaction. Even under the conditions of civilization we are often acutely sensitive to this fact, while under the conditions of primitive life, in imperfect shelters, caves or tents, where no other source of artificial light and heat is known, the satisfaction is immensely greater. At the same time fire is associated with food, it is a protection from wild beasts and the accompaniment of the festival. It may even take on a sacred and symbolic character, and the Roman goddess Vesta was, as Ovid said, simply ‘living flame.’
While fruit or fire would tend to make the emotional tone of red pleasant, another very powerful factor in its emotional influences, though this time as much by causing terror as pleasure, is the fact that it is the color of blood. That ‘the blood is the life’ is a belief instinctively stamped even on the emotions of animals, and it has not died even in civilized man, for the sight of blood produces on many persons a sickening and terrifying sensation which is only overcome by habit and experience or by a very strong effort of will. It is not surprising that in some parts of the world, and even in our own Indo-European group of languages, the name for red is ‘blood-color.’
It is evident, however, that at a very early period of primitive culture the blood had ceased to be merely a source of terror, or even of the joy of battle. We find everywhere that blood is blended into complex ritual customs, and thus associated with complex emotional states. Among the ancient Arabians blood was smeared on the body on various occasions, and in modern Arabia blood is still so used. Everywhere, even in the folk-lore of modern Europe, we find that blood is a medicine, as it is also among the primitive aborigines of Australia, so carefully investigated by Baldwin Spencer and Gillen. Among these latter primitive people we meet with a phenomenon of very great significance. We find, that is, that blood is the earliest pigment. There can be little doubt that the earliest paint used by man—no doubt by man when in a much more primitive condition than even the Australians—was blood. In the initiation rites of the Arunta tribes, asdescribed by Spencer and Gillen, the chief performer is elaborately decorated with patterns in eagle-hawk down stuck to his body with blood drawn from some member of the tribe. It was estimated that one man alone, on one of these occasions, allowed five half-pints to be taken from him during a single day; at the same time the blood is not regarded as sufficient pigment and the down is also colored red and yellow with ochre. Red ochre, Spencer and Gillen remark, is frequently a substitute for blood or is used with it. Blood is a medicine, and when any one is ill he is first rubbed over with red ochre, it being obvious to the primitive mind that the ochre will share the remedial properties of blood; in the same way ceremonial objects may sometimes be rubbed over with ochre instead of blood. They associate this red ochre especially with women’s blood; and it is said that once some women after long walking were so exhausted that hemorrhage came on and this gave rise to deposits of red ochre. Other red ochre pits, also, they attribute to blood which flowed from women. It appears also that the blood with which sacred implements used in the ritual ceremonies of these Central Australians were smeared must be drawn from women.
Far from Australia, among the hill tribes of the Central Indian hills, we find the same blood ritual and the same tendency to substitute pigments for blood. Among some of the Bengal tribes, says Crooke, blood is drawn from the husband’s little finger, mixed with betel and eaten by the bride. A further stage is seen among the allied Kurmis who mix the blood with lac dye. Lastly come the rites, common to all these tribes, in which the bridegroom, often in secrecy, covered by a sheet, rubs vermilion on the parting of the girl’s hair, while the women relations smear their toes with lac dye. It is a sacramental rite, and after the husband’s death the widow solemnly washes off the red from her hair, or flings the little box in which she keeps the coloring matter into running water.
Some of the foregoing facts, both in Australia and India, suggest the transition to another factor in the emotional potency possessed by red. Red is not only the color of fire and of war and of ritual pigment; it is the color of love. This is certainly an ancient and powerful factor in the emotional attitude towards red. Secondary sexual characters, even among birds, are often red; many fishes, also, at the epoch of the oviposit show a red tint on the orifice of the sexual apparatus; patches of red, sometimes very brilliant, but only appearing when the animal is mature, are perhaps the commonest adornments of monkeys. In man the color of the hair and beard, the most conspicuous of the secondary sexual characters, is most usually brown, or some other variety of red. The lips are crimson, the mucous membrane generally a dark red; the scarlet of the blush, among all fair races, whateverother sources it may have, is always regarded as especially the ensign of love. The rose is the flower of love, as the pale lily is of virtue. This association is quite inapt, and many people who are sensitive in such matters feel that the lily and many white flowers are far more symbolical of rapture and voluptuousness than the rose. It is, however, the color and not the scent or other qualities that has exerted decisive influence on the choice of the symbol. In the Teutonic symbolism of fourteenth century Europe red was the color of love, as also, with yellow, it was the favorite color for garments. In more modern times this last tendency has survived. Sardou decides, it is reported, the color of the dresses to be worn in his plays, on the ground that if he did not the actresses would all wear red to attract attention to themselves, as once occurred at the Odéon. Eighteen hundred years earlier, Clement of Alexandria had written: “Would it were possible to abolish purple in dress, so as not to turn the eyes of the spectators on the faces of those that wear it!” He proceeds to lament that women make all their garments of purple (the classic purple was really a red) in order to inflame lust—those ‘stupid and luxurious purples’ which have caused Tyre and Sidon and the Lacedæmonian Sea to be so much in demand for their purple fishes. Similar phenomena are noted on the other side of the world. Thus the Japanese, as the Rev. Walter Weston informs us, have a proverb: ‘Love flies with a red petticoat.’ Married women are not there supposed to wear red petticoats, for they are too attractive, and a married woman should be attractive only to her husband. The æsthetic Japanese may be thought to be specially sensitive to color, but in Africa also, in Loango, as Pechuel-Loesche mentions, pregnant women are forbidden to wear red, and it would doubtless be possible to find many similar indications of this feeling in other parts of the world.
We have now passed in review all the influences which, by force of their powerful attraction or repulsion, have during countless ages impressed on man, and often on his ancestors, the strong and poignant emotions which accompany the sensation of the most vividly and persistently seen of all colors. We find evidence of the reality of the influences we have traced—especially those of fire, blood and love—in Christian ecclesiastical symbolism, according to which red variously signifies ardent love, burning zeal, energy, courage, cruelty and bloodthirstiness. To the antagonism and complexity of these influences we must doubtless attribute the disturbing nature of the emotion aroused by the group of red sensations and the fluctuations in the predilection felt towards it. It is at once the most attractive and the most repulsive of colors. To enjoy it we must use it economically. The vision of poppies on a background of golden corn, the glint of roses embowered in green leaves, the sudden flash of a scarlet flower on a southernwoman’s dark hair—it is in such visions as these that red gives us its emotional thrill altogether untouched by pain. If the ‘multitudinous seas’ were indeed ‘incarnadined’ for us in ‘one red,’ if the sky were scarlet, or all vegetation crimson, the horror of the world would be painful to contemplate for nervous systems moulded to our vision of nature. Our eyes have developed in a world where the green and blue rays meet us at every step, and where we have in consequence been almost as dulled to them as we are to the weight of the atmosphere that presses in on us on every side. It is under the clouded skies of northern lands that blue is counted the loveliest of colors; it is in the desert that green becomes supremely beautiful and sacred.