figure 82.
figure 82.
5.On the space-penetrating power of telescopes.—The power of telescopes to penetrate into the profundity of space is the result of the quantity of light they collect and send to the eye in a state fit for vision. This property of telescopes is sometimes designated by the expressionIlluminating Power.
Sir W. Herschel appears to have been the first who made a distinction between themagnifyingpower, and thespace-penetratingpower of a telescope; and there are many examples whichprove that such a distinction ought to be made, especially in the case of large instruments. For example, the small star, or speck of light, which accompanies the pole-star, may be seen through a telescope of large aperture, with a smaller magnifying power than with a telescope of a small aperture furnished with a much higher power. If the magnifying power is sufficient to show the small star completely separated from the rays which surround the large one, this is sufficient in one point of view; but in order that this effect may be produced, so as to render the small star perfectly distinguishable, a certain quantity of light must be admitted into the pupil of the eye—which quantity depends upon the area of the object-glass or speculum of the instrument, or, in other words, on the illuminating power. If we compare a telescope of 2¾ inches aperture with one of 5 inches aperture, when the magnifying power of each does not exceed 50 times for terrestrial objects, the effect of illuminating power is not so evident; but if we use a power of 100 for day objects, and 180 for the heavenly bodies, the effects of illuminating power is so clearly perceptible, that objects not only appear brighter, and more clearly visible, in the larger telescope, but with the same magnifying power, they alsoappearlarger, particularly when the satellites of Jupiter and small stars are the objects we are viewing.
Sir W. Herschel remarks, that ‘objects are viewed in their greatest perfection, when, in penetrating space, the magnifying power is so low as only to be sufficient to show the object well—and when, in magnifying objects, by way of examining them minutely, the space-penetrating power is no higher than what will suffice for the purpose; for in the use of either power, the injudicious overchargeof the other will prove hurtful to vision.’ When illuminating power is in too high a degree, the eye is offended by the extreme brightness of the object. When it is in too low a degree, the eye is distressed by its endeavours to see what is beyond its reach; and therefore it is desirable, when we wish to give the eye all the assistance possible, to have the illuminating and the magnifying powers in due proportion. What this proportion is, depends, in a certain degree, upon the brightness of the object. In proportion to its brightness or luminosity, the magnifying power may, to a certain extent, be increased. Sir W. Herschel remarks, in reference to α Lyræ, ‘This star, I surmise, has light enough to bear being magnified, at least a hundred thousand times, with no more than six inches of aperture.’ However beautifully perfect any telescopes may appear, and however sharp theirdefiningpower, their performance is limited by their illuminating powers—which are as the squares of the diameters of the apertures of the respective instruments. Thus, a telescope whose object-glass is 4 inches diameter will have four times the quantity of light, or illuminating power, possessed by a telescope whose aperture is only 2 inches, or in the proportion of 16 to 4,—the square of 4 being 16, and the square of 2 being 4.
The nature of thespace-penetrating power, to which we are adverting, and the distinction between it, and magnifying power, may be illustrated from a few examples taken from Sir W. Herschel’s observations.
The first observation which I shall notice refers to thenebulabetween η and ζ Ophiuchi, discovered by Messier in 1764. The observation was made with a 10 feet reflector, having a magnifyingpower of 250, and a space-penetrating power of 28.67. His note is dated May 3, 1783. ‘I see several stars in it, and make no doubt a higher power and more light will resolve it all into stars. This seems to me a good nebula for the purpose of establishing the connection between nebulæ and clusters of stars in general.’—‘June 18, 1784. The same nebula viewed with a Newtonian 20 feet reflector; penetrating power 61, and a magnifying power of 157; a very large and a very bright cluster of excessively compressed stars. The stars are but just visible, and are of unequal magnitudes. The large stars are red, the cluster is a miniature of that near Flamstead’s forty-second Comæ Berenices; Right ascension 17h6m32sPolar distance 108° 18´´’ In this case, a penetrating power of about 28, with a magnifying power of 250, barely shewed a few stars; when in the second instrument the illuminating power of 60 with the magnifying power of only 157 showed them completely.
Subsequently to the date of the latter observation, the 20 feet Newtonian telescope was converted into an Herschelian instrument, by taking away the small speculum, and giving the large one the proper inclination for obtaining the front view; by which alteration the illuminating power was increased from 61 to 75, and the advantage derived from the alteration was evident in the discovery of the satellites of Uranus by the altered telescope, which before was incompetent in the point of penetration, or illuminating power. ‘March 14, 1798, I viewed the Georgian planet (or Uranus) with a new 25 feet reflector. Its penetrating power is 95.85, and having just before also viewed it with my 20 feet instrument, I found that with an equal magnifying power of 300, the 25 feettelescope had considerably the advantage of the former.’ The aperture of the 20 feet instrument was 18.8 inches, and that of the 25 feet telescope, 24 inches, so that the superior effect of the latter instrument must have been owing to its greater illuminating power. The following observations show the superior power of the 40 feet telescope as compared with the 20 feet.—‘Feb. 24, 1786, I viewed the nebula near Flamstead’s fifth Serpentis, with my 20 feet reflector, magnifying power 157. The most beautiful extremely compressed cluster of small stars; the greatest part of them gathered together into one brilliant nucleus, evidently consisting of stars, surrounded with many detached gathering stars of the same size and colour. R.A. 15h7m12s. P.D. 87° 8´´’—‘May 27, 1791, I viewed the same object with my 40 feet telescope, penetrating power 191.69, magnifying power 370. A beautiful cluster of stars. I counted about 200 of them. The middle of it is so compressed, that it is impossible to distinguish the stars.’—‘Nov. 5, 1791, I viewed Saturn with the 20 and 40 feet telescopes.Twenty feet.The fifth satellite of Saturn is very small. The first, second, third, fourth and fifth, and the new sixth satellites are in their calculated places.Forty feet.I see the new sixth satellite much better with this instrument than with the 20 feet. The fifth is also much larger here than in the 20 feet, in which it was nearly the same size as a small fixed star, but here it is considerably larger than that star.’
These examples, and many others of a similar kind, explain sufficiently the nature and extent of that species of power that one telescope possesses over another, in consequence of its enlarged aperture; but the exact quantity of this power is in some degree uncertain. To ascertain practically the illuminating power of telescopes, we must trythem with equal powers on such objects as the following,—the small stars near the pole-star, and near Rigel and ε Bootis—the division in the ring of Saturn—and distant objects in the twilight or towards the evening. These objects are distinctly seen with a 5 feet achromatic of 38/10inches aperture, and an illuminating power of 144, while they are scarcely visible in a 3½ feet with an aperture of 2¾ inches, and an illuminating power of 72, supposing the same magnifying power to be applied. The illuminating power of a telescope is best estimated, in regard to land objects, when it is tried on minute objects, and such as are badly lighted up; and the advantage of a telescope with a large aperture will be most obvious, when it is compared with another of inferior size in the close of the evening, when looking at a printed bill composed of letters of various sizes. As darkness comes on, the use of illuminating power becomes more evident. In a 5 feet telescope some small letters will be legible, which are hardly discernible in the 3½ feet, and in the 2½ feet are quite undefinable, though the magnifying powers be equal. Sir W. Herschel informs us, that in the year 1776, when he had erected a telescope of 20 feet focal length of the Newtonian construction, one of its effects by trial was, that when towards evening, on account of darkness, the natural eye could not penetrate far into space, the telescope possessed that power sufficiently to show, by the dial of a distant church steeple, what o’clock it was, notwithstanding the naked eye could no longer see the steeple itself.
In order to convey an idea of thenumbersby which the degree of space-penetrating power is expressed, and the general grounds on which they rest, the following statements may be made. The depth to which the naked eye can penetrate into thespaces of the heavens, is considered as extending to the twelfth order of distances—in other words, it can perceive a star at a distance 12 times farther than those luminaries, such as Sirius, Arcturus or Capella, which, from their vivid light, we presume to be nearest to us. It has been stated above, that Herschel calculated his 10 feet telescope to have a space-penetrating power of 28.67, that is, it could enable us to descry a star 28 times farther distant than the naked eye can reach. His 20 feet Newtonian was considered as having a similar power of 61; his 25 feet, nearly 96, and his 40 feet instrument, a power of 191.69. If each of these numbers be multiplied by 12, the product will indicate how much farther these telescopes will penetrate into space than the nearest range of the fixed stars, such as those of the first magnitude. For instance, the penetrating power of the 40 feet reflector being 191.69, this number multiplied by 12, gives a product of 2,300, which shows, that were there a series of two thousand three hundred stars extended in a line beyond Sirius, Capella and similar stars—each star separated from the one beyond it, by a space equal to the distance of Sirius from the earth—they might be all seen through the 40 feet telescope. In short, the penetrating power of telescopes is a circumstance which requires to be particularly attended to in our observations of celestial phenomena, and in many cases, is of more importance thanmagnifyingpower. It is the effect produced by illuminating power that renders telescopes, furnished with comparatively small magnifying powers, much more efficient in observing comets and certain nebulæ and clusters of stars, than when high powers are attempted. Every telescope may be so adjusted, as to produce different space-penetratingpowers. If we wish to diminish such a power, we have only to contract the object-glass or speculum, by placing circular rims, or apertures of different degrees of breadth, across the mouth of the great tube of the instrument. But we cannot increase this illuminating power beyond a certain extent, which is limited by the diameter of the object-glass. When we wish illuminating power beyond this limit, we must be furnished with an object-glass or speculum of a larger size; and hence, the rapid advance in price of instruments which have large apertures, and consequently high illuminating powers. Mr. Tulley’s 3½ feet achromatics of 2¾ inches aperture, sell at £26 5s. When the aperture is 3¼ inches, the price is £42. When 3¾ inches, £68 5s. The following table contains a statement of the ‘comparative lengths, apertures, illuminating powers, and prices, of Achromatic Refractors, and Gregorian Reflectors,’ according to Dr. Kitchener.
The illuminating powers stated in the above table are only comparative. Fixing on the number 25 as the illuminating power of a 2 feet telescope, 16/10inch aperture, that of a 2½ feet 2 inchesinches aperture, will be 40, of a 5 feet 38/10inch aperture, 144, &c. If the illuminating power of a Gregorian 1½ foot, and 3 inches aperture, be 90, a 5 feet, with 9 inches aperture, will be 810, &c.
6.On choosing Telescopes, and ascertaining their properties.
It is an object of considerable importance, to every astronomical observer, that he should be enabled to form a judgment of the qualities of his telescope, and of any instruments of this description which he may intend to purchase. The following directions may perhaps be useful to the reader in directing him in the choice of an achromatic refracting telescope.
Supposing that an achromatic telescope of 3½ feet focal length, and 3¼ inches aperture were offered for sale, and that it were required to ascertain whether the object-glass, on which its excellence chiefly depends—is a good one and duly adjusted;—some opinion may be formed by laying the tube of the telescope in a horizontal position, on a firm support, about the height of the eye,—and by placing a printed card or a watch glass vertically, but in an inverted position, against some wall or pillar, at 40 or 50 yards distant, so as to be exposed to a clear sky. When the telescope is directed to this object, and accurately adjusted to the eye—should the letters on the card, or the strokes and dots on the watch-glass appear clearly and sharply defined, without any mistiness or coloration, and if very small points appear well defined—great hopes may be entertained that the glass will turn out a good one. But a telescope may appear a good one, when viewing common terrestrial objects, to eyes unaccustomed to discriminate deviations from perfect vision, while itmay turn out to be an indifferent one, when directed to certain celestial objects. Instead therefore of a printed card, fix a black board, or one half of a sheet of black paper, in a vertical position at the same distance, and a circular disk of white writing paper, about ¼ of an inch in diameter, on the centre of the black ground. Then having directed the telescope to this object, and adjusted for the place of distinct vision, mark with a black-lead pencil the sliding eye-tube, at the end of the main tube, so that this position can always be known; and if this sliding tube be gradually drawn out, or pushed in, while the eye beholds the disk, it will gradually enlarge and lose its colour, till its edges cease to be well-defined. Now, if the enlarged misty circle is observed to be concentric with the disk itself, the object-glass is properly centered, as it has reference to the tube; but if the misty circle goes to one side of the disk, the cell of the object-glass is not at right angles to the tube, and must have its screws removed and its holes elongated, by a rattailed file, small enough to enter the holes. When this has been done, the cell may be replaced, and the disk examined a second time, and a slight stroke on one edge of the cell, by a wooden mallet, will show by the alteration made in the position of the misty portion of the disk, how the adjustment is to be effected, which is known to be right when a motion in the sliding tube will make the diluted disk enlarge in a circle concentric with the disk itself. When the disk will enlarge so as to make a ring of diluted white light round its circumference, as the sliding tube holding the eye-piece is pushed in or drawn out, the cell may be finally fixed by the screws passing through its elongated holes.
When the object-glass is thus adjusted, it may then be ascertained whether the curves of the respective lenses composing the object-glass are well-formed and suitable for each other. If a small motion of the sliding tube of about1/10th of an inch in a 3½ feet telescope, from the point of distinct vision, will dilute the light of the disk and render the appearance confused, the figure of the object-glass is good; particularly if the same effect will take place at equal distances from the point of distinct vision, when the tube is alternately drawn out and pushed in. A telescope that will admit of much motion in the sliding tube without sensibly affecting the distinctness of vision, will not define an object well at any point of adjustment, and must be considered as having an imperfect object-glass, inasmuch as the spherical aberration of the transmitted rays is not duly corrected. The due adjustment of the convex lens, or lenses, to the concave one, will be judged of by the absence of coloration round the enlarged disk, and is a property distinct from the spherical aberration; the achromatism depending on the relative focal distances of the convex and concave lenses, is regulated by the relative dispersive powers of the pieces of glass made use of; but the distinctness of vision depends on a good figure of the computed curves that limit the focal distances. When an object-glass is free from imperfection in both these respects, it may be called a good glass for terrestrial purposes.
It still, however, remains to be determined how far such an object-glass may be good for viewing a star or a planet, and can only be known by actual observations on the heavenly bodies. When a good telescope is directed to the moon or to Jupiter, the achromatism may be judged of, byalternately pushing in, and drawing out the eye-piece, from the place of distinct vision. In the former case, a ring of purple will be formed round the edge; and in the latter, a ring of light green, which is the central colour of the prismatic spectrum; for these appearances show, that the extreme colours red and violet are corrected. Again, if one part of a lens employed have a different refractive power from another part of it, that is, if the flint-glass particularly is not homogeneous, a star of the first and even of the second magnitude will point out the natural defect by the exhibition of an irradiation, or what is calleda wing, at one side, which no perfection of figure or of adjustment will banish, and the greater the aperture the more liable is the evil to happen. Hence caps with different apertures are usually supplied with large telescopes, that the extreme parts of the glass may be cut off, in observations requiring a round and well-defined image of the body observed.
Another method of determining the figure and quality of an object-glass is by first covering its centre by a circular piece of paper, as much as one half of its diameter, and adjusting it for distinct vision of a given object, such as the disk above mentioned, when the central rays are intercepted—and then trying if the focal length remains unaltered when the paper is taken away, and an aperture of the same size applied, so that the extreme rays may in their turn be cut off. If the vision remains equally distinct in both cases, without any new adjustment for focal distance, the figure is good, and the spherical aberration cured, and it may be seen by viewing a star of the first magnitude successively in both cases, whether the irradiation is produced more by the extreme orby the central parts of the glass. Or, in case the one half be faulty and the other good, a semicircular aperture, by being turned gradually round in trial, will detect what semicircle contains the defective portion of the glass; and if such portion should be covered, the only inconvenience that would ensue, would be the loss of so much light as is thus excluded. When an object-glass produces radiations in a large star, it is unfit for the nicer observations of astronomy, such as viewing double stars of the first class. The smaller a large star appears in any telescope, the better is the figure of the object-glass, but if the image of the star be free from wings, the size of its disk is not an objection in practical observations.30
Some opticians are in the habit of inserting a diaphragm into the body of the large tube, to cut off the extreme rays coming from the object-glass when the figure is not good, instead of lessening the aperture by a cap. When this is the case, a deficiency of light will be the consequence beyond what the apparent aperture warrants. It is therefore proper to examine that the diaphragm be not placed too near the object-glass, so as to intercept any of the useful rays. Sometimes a portion of the object-glass is cut off by the stop in the eye-tube. To ascertain this, adjust the telescope to distinct vision, then take out the eye-glasses, and put your finger on some other object on the edge of the outside of the object-glass, and look down the tube; if you can see the tip of your finger, or any object in its place, just peeping over the edge of the object-glass, no part is cut off. I once had a 3½ feet telescope whose object-glass measured 3inches diameter, which was neither so bright, nor did it perform in other respects nearly so well as another of the same length whose object-glass was only 2¾ inches diameter; but I found that a diaphragm was placed about a foot within the end of the large tube, which reduced the aperture of the object-glass to less than 2½ inches; and when it was removed the telescope was less distinct than before. The powers given along with this instrument were much lower than usual—none of them exceeding 100 times. This is a trick not uncommon with some opticians.
Dr. Pearson mentions that an old Dollond’s telescope of 63 inches focal length, and 3¾ inches aperture, supposed to be an excellent one, was brought to Mr. Tulley, when he was present, and the result of the examination was that its achromatism was not perfect. The imperfection was thus determined by experiment. A small glass globe was placed at 40 yards distance from the object-end of the telescope when the sun was shining, and the speck of light seen reflected from this globe formed a good substitute for a large star, as an object to be viewed. When the focal length of the object-glass was adjusted to this luminous object, no judgment could be formed of its prismatic aberrations, till the eye-piece had been pushed in beyond the place of correct vision; but when the telescope was shortened a little, the luminous disk occasioned by such shortening was strongly tinged withredrays at its circumference. On the contrary, when the eye-piece was drawn out, so as to lengthen the telescope too much, the disk thus produced was tinged with a small circle ofredat itscentre, thereby denoting that the convex lens had too short a focal length; and Mr. Tulley observed, that if one or both of the curvesof the convex lens were flattened till the total focal length should be about 4 inches increased, it would render the telescope quite achromatic, provided in doing this the aberration should not be increased.
The following general remarks may be added. 1. To make anything like an accurate comparison of telescopes, they must be tried not only at the same place, but as nearly as possible at the same time, and, if the instruments are of the same length and construction, if possible, with the same eye-piece. 2. A difference of 8 or 10 times in the magnifying power, will sometimes, on certain objects, give quite a different character to a telescope. It has been found by various experiments that object-glasses of two or three inches longer focus will produce different vision with the same eye-piece. 3. Care must be taken to ascertain that the eye-glasses are perfectly clean and free from defects. The defects of glass are either from veins—specks—scratches—colour, or an incorrect figure. To discover veins in an eye or an object-glass, place a candle at the distance of 4 or 5 yards; then look through the glass, and move it from your eye till it appear full of light—you will then see every vein, or other imperfection in it which may distort the objects and render vision imperfect. Specks or scratches, especially in object-glasses, are not so injurious as veins, for they do not distort the object, but only intercept a portion of the light. 4. We cannot judge accurately of the excellence of any telescope by observing objects with which we are not familiarly acquainted. Opticians generally try an instrument at their own marks, such as the dial-plate of a watch, a finely engraved card, a weather-cock, or the moon and the planet Jupiter, when nearthe meridian. Of several telescopes of the same length, aperture and magnifying power, that one is generally considered the best with which we can read a given print at the greatest distance, especially if the print consists offigures, such as a table of logarithms, where the eye is not apt to be deceived by the imagination, inguessingat the sense of a passage, when two or three words are distinguished.
There is a circumstance which I have frequently noticed, in reference to achromatic telescopes, particularly those of a small size, and which I have never seen noticed by any optical writer. It is this,—if the telescope, when we are viewing objects, be gradually turned round its axis, there is a certain position in which the objects will appear distinct and accurately defined; and if it be turned round exactly a semicircle from this point, the same degree of distinctness is perceived; but in all other positions, there is an evident want of clearness and defining power. This I find to be the case in more than ten 1 foot and 2 feet telescopes now in my possession; and therefore I have put marks upon the object-end of each of them, to indicate the positions in which they should be used for distinct observation.—This is a circumstance which requires, in many cases, to be attended to in the choice and the use of telescopical instruments, and in fixing and adjusting them on their pedestals. In some telescopes this defect is very striking, but it is in some measure perceptible in the great majority of instruments which I have had occasion to inspect. Even in large and expensive achromatic telescopes this defect is sometimes observable. I have an achromatic whose object-glass is 41/10inches diameter, which was much improved in its defining power,by being unscrewed from its original position, or turned round its axis—about one-eighth part of its circumference. This defect is best detected by looking at a large printed bill, or a sign-post at a distance, when, on turning round the telescope or object-glass, the letters will appear much better defined in one position than in another. The position in which the object appears least distinct is when the upper part of the telescope is a quadrant of a circle different from the two positions above-stated, or at an equal distance from each of them.
7.On the mode of determining the magnifying power of Telescopes.
In regard to refracting telescopes, we have already shown that, when a single eye-glass is used, the magnifying power may be found by dividing the focal distance of the object-glass by that of the eye-glass. But when a Huygenian eye-piece, or a four-glass terrestrial eye-piece such as is now common in achromatic telescopes, is used, the magnifying power cannot be ascertained in this manner; and in some of the delicate observations of practical astronomy, it is of the utmost importance to know the exact magnifying power of the instrument with which the observations are made, particularly when micrometrical measurements are employed to obtain the desired results.—The following is a general method of finding the magnifying powers of telescopes when the instrument called adynameteris not employed; and it answers for refracting and reflecting telescopes of every description.
Having put up a small circle of paper, an inch or two in diameter, at the distance of about 100 yards, draw upon a card 2 black parallel lines, whose distance from each other is equal to thediameter of the paper circle. Then view through the telescope the paper circle with one eye, and the parallel lines with the other; and let the parallel lines be moved nearer to or further from, the eye, till they seem exactly to cover the small circle viewed through the telescope. The quotient obtained by dividing the distance of the paper circle by the distance of the parallel lines from the eye, will be the magnifying power of the telescope. It requires a little practice before this experiment can be performed with accuracy. The one eye must be accustomed to look at an object near at hand, while the other is looking at a more distant object through the telescope. Both eyes must be open at the same time, and the image of the object seen through the telescope must be brought into apparent contact with the real object near at hand. But a little practice will soon enable any observer to perform the experiment with ease and correctness, if the telescope be mounted on a firm stand, and its elevation or depression produced by rack-work.
The following is another method, founded on the same principle:—Measure the space occupied by a number of the courses, or rows of bricks in a modern building—which, upon an average, is found to have 8 courses in 2 feet, so that each course or row, is 3 inches. Then cut a piece of paper 3 inches in height, and of the length of a brick—which is about 9 inches—so that it may represent a brick, and fixing the paper against the brick wall, place the telescope to be examined at the distance of about 80 or 100 yards from it. Now, looking through the telescope at the paper with one eye, and at the same time, with the other eye, looking past the telescope, observe what extent of wall the magnified image of the paper appears tocover, then count the courses of bricks in that extent, and it will give the magnifying power of the telescope. It is to be observed, however, that the magnifying power determined in this way, will be a fraction greater than for very distant objects, as the focal distance of the telescope is necessarily lengthened in order to obtain distinct vision of near objects.
In comparing the magnifying powers of two telescopes, or of the same telescope, when different magnifying powers are employed, I generally use the following simple method. The telescopes are placed at 8 or 10 feet distant from a window, with their eye-ends parallel to each other, or at the same distance from the window. Looking at a distant object, I fix upon a portion of it whose magnified image will appear to fill exactly two or three panes of the window. Then putting on a different power, or looking through another telescope, I observe the same object, and mark exactly the extent of its image on the window-panes, and compare the extent of the one image with the other. Suppose for example, that the one telescope has been previously found to magnify 90 times, and that the image of the object fixed upon exactly fills three panes of the window, and that with the other power or the other telescope, the image fills exactly two panes, then the magnifying power is equal to two thirds of the former, or 60 times; and were it to fill only one pane, the power would be about 30 times. A more correct method is to place at one side of the window, a narrow board, two or three feet long, divided into 15 or 20 equal parts, and observe how many of these parts appear to be covered by the respective images, of the different telescopes. Suppose, in the one case, 10 divisions to be covered by the image, in a telescope magnifying 90 times, andthat the image of the same object in another telescope, measures 6 divisions, then its power is found by the following proportion, 10 : 90 : 6 : 54 : that is, this telescope magnifies 54 times.
Another mode which I have used for determining, to a near approximation, the powers of telescopes, is as follows:—Endeavour to find the focus of a single lens which is exactly equivalent to the magnifying power of the eye-piece, whether the Huygenian or the common terrestrial eye-piece. This may be done by taking a small lens, and using it as an object-glass to the eye-piece. Looking through the eye-piece to a window and holding the lens at a proper distance, observe whether the image of one of the panes exactly coincides with the pane, as seen by the naked eye; if it does, then the magnifying power of the eye-piece is equal to that of the lens. If the lens be ½ inch focal length, the eye-piece will produce the same magnifying power, as a single lens when used as an eye-glass to the telescope, and the magnifying power will then be found by dividing the focal distance of the object-glass by that of the eye-glass. But if the image of the pane of glass does not exactly coincide with the pane as seen by the other eye, then proportional parts may be taken by observing the divisions of such a board as described above, or we may try lenses of different focal distances. Suppose, for example, that a lens 2 inches focal length had been used, and that the image of a pane covered exactly the space of two panes, the power of the eye-piece is then equal to that of a single lens 1 inch focal distance.
The following is another mode depending on the same general principle. If a slip of writing-paper one inch long, or a disk of the same material of one inch diameter, be placed on a black groundat from 30 to 50 yards distance from the object-end of the telescope, and a staff painted white, and divided into inches and parts by strong black lines, be placed vertically near the said paper or disk; the eye that is directed through the telescope when adjusted for vision, will see the magnified disk, and the other eye, looking along the outside of the telescope, will observe the number of inches and parts that the disk projected on it will just cover, and as many inches as are thus covered will indicate the magnifying power of the telescope—at the distance for which it is adjusted for distinct vision. The solar power, or powers for very distant objects, may be obtained by the following proportion:—As the terrestrial focal length, at the given distance: is to the solar focal length :: so is the terrestrial power, to the solar power. For example, a disk of white paper one inch in diameter, was placed on a black board, and suspended on a wall contiguous to a vertical black staff that was graduated into inches by strong white lines, at a distance of 33 yards 2½ feet, and when the adjustment for vision was made with a 42 inch telescope, the left eye of the observer viewed the disk projected on the staff, while the right eye observed that the enlarged image of the disk covered just 58½ inches on the staff, which number was the measure of the magnifying power, at the distance answering to 33 yards 2½ feet—which in this case exceeded the solar focus by an inch and a half. Then according to the above analogy, we have, as 43.5 : 42 :: 58.5 : 56.5 nearly. Hence the magnifying power due to the solar focal length of the telescope in question is 56.5, and the distance 33 yards 2½ feet, is that which corresponds to an elongation of the solar focal distance aninch and a half.31If we multiply the terrestrial and the solar focal distances together, and divide the product by their difference, we shall again obtain the distance of the terrestrial object from the telescope. Thus, (43.5 + 42)/1.5 = 1218 inches = 101.5 feet, or 33 yards 2½ feet.
The magnifying power of a telescope is also determined, by measuring the image which the object-glass or the large speculum of a telescope forms at its solar focus. This is accomplished by means of an instrument called aDynameter. This apparatus consists of a strip of mother-of-pearl, marked with equal divisions, from the1/100th to the1/1000th of an inch apart, according to the accuracy required. This measure is attached to a magnifying lens in its focus, in order to make the small divisions more apparent. When the power of a telescope is required, the person must measure the clear aperture of the object-glass, then holding the pearldynameternext the eye-glass, let him observe how many divisions the small circle of light occupies, when the instrument is directed to a bright object. Then by dividing the diameter of the object-glass by the diameter of this circle of light, the power will be obtained.32The most accurate instrument of this kind is theDouble Image Dynameterinvented by Ramsden, and another on the same principle now made by Dollond, a particular description of which may be found in Dr. Pearson’s ‘Introduction to Practical Astronomy.’ The advantage attending these dynameters is that they do not require any knowledge of the thickness and focal lengths of any of the lenses employed in a telescope, nor yet of theirnumber or relative positions; neither does it make any difference whether the construction be refracting or reflecting, direct or inverting. One operation includes the result arising from the most complicated construction.
I shall only mention farther the following method of discovering the magnifying power, which is founded on the same general principle as alluded to above. Let the telescope be placed in such a position opposite the sun, that the rays of light may fall perpendicularly on the object-glass; and the pencil of rays may be received on a piece of paper, and its diameter measured. Then, as the diameter of the pencil of rays is to that of the object-glass, so is the magnifying power of the telescope.
8.—On cleaning the lenses of telescopes.—
It is necessary, in order to distinct vision, that the glasses, particularly the eye-glasses of telescopes be kept perfectly clean, free of damp, dust, or whatever may impede the transmission of the rays of light. But great caution ought to be exercised in the wiping of them, as they are apt to be scratched, or otherwise injured by a rough and incautious mode of cleaning them. They should never be attempted to be wiped unless they really require it; and, in this case, they should be wiped carefully and gently with a piece of new and soft lamb’s-skin leather. If this be not at hand, a piece of fine silk paper, or fine clean linen may be used as a substitute. The lens which requires to be most particularly attended to is the second glass from the eye, or the field-glass; for if any dust or other impediment be found upon this glass, it is always distinctly seen, being magnified by the glass next the eye. The next glass which requires attention is the fourthfrom the eye, or that which is next the object. Unless the glass next the eye be very dusty, a few small spots or grains of dust are seldom perceptible. The object-glass of an achromatic should seldom be touched, unless damp adheres to it. Care should be taken never to use pocket handkerchiefs or dirty rags for wiping lenses. From the frequent use of such articles, the glasses of seaman’s telescopes get dimmed and scratched in in the course of a few years. If the glasses be exceedingly dirty, and if greasy substances are attached to them, they may be soaked in spirits and water, and afterwards carefully wiped. In replacing the glasses in their socket, care should be taken not to touch the surfaces with the fingers, as they would be dimmed with the perspiration: they should be taken hold of by the edges only, and carefully screwed into the same cells from which they were taken.
It appears to have been almost overlooked by opticians and others, that telescopes may be constructed so as to exhibit a beautiful and minute view of very near objects, and to produce even a microscopic effect, without the least alteration in thearrangementof the lenses of which they are composed. This object is effected simply by making the eye-tube of a telescope of such a length as to be capable of being drawn out 12 or 13 inches beyond the point of distinct vision for distant objects. The telescope is then rendered capable of exhibiting with distinctness all kinds of objects, from the most distant to those which areplaced within 3 or 4 feet of the instrument—or not nearer than double the focal distance of the object-glass. Our telescopes, however, are seldom or never fitted with tubes that slide farther than an inch or two beyond the point of distinct vision for distant objects, although a tube of a longer size than usual, or an additional tube would cost but a very trifling expence.
The following, among many others, are some of the objects on which I have tried many amusing experiments with telescopes fitted up with the long tubes to which I allude. The telescope to which I shall more particularly advert is an achromatic, mounted on a pedestal, having an object-glass about 19 inches focal length, and 1⅝ inch diameter, with magnifying powers for distant objects of 13 and 20 times. When this instrument is directed to a miniature portrait, 3½ inches in length, placed in a good light, at the distance of about 8 or 10 feet, it appears as large as an oil-painting four or five feet long, and represents the individual as large as life. The features of the face appear to stand out in bold relief: and perhaps there is no representation of the human figure that more resembles the living prototype, than in this exhibition, provided the miniature is finely executed. In this case the tube requires to be pulled out four or five inches from the point of distinct vision for distant objects, and consequently the magnifying power is proportionally increased. Another class of objects to which such a telescope may be applied isPerspective prints, either of public buildings, streets or landscapes. When viewed in this way they present a panoramic appearance, and seem nearly as natural as life—just in the same manner as they appear in the Optical Diagonal Machine, or when reflected in a largeconcave mirror—with this advantage, that, while in these instruments the left hand side of the print appears where the right should be,—the objects seen through the telescope appear exactly in their natural position. In this case, however, the telescope should have a small magnifying power, not exceeding 5 or 6 times, so as to take in the whole of the landscape. If an astronomical eye-piece be used, the print will require to be inverted.
Other kinds of objects which may be viewed with this instrument, are trees, flowers, and other objects in gardens immediately adjacent to the apartment in which we make our observations. In this way we may obtain a distinct view of a variety of rural objects, which we cannot easily approach, such as the buds and blossoms on the tops of trees, and the insects with which they may be infested. There are certain objects on which the telescope may be made to produce a powerful microscopical effect, such as the more delicate and beautiful kinds of flowers, the leaves of trees, and similar objects. In viewing such objects, the telescope may be brought within little more than double the focal distance of the object-glass from the objects to be viewed, and then the magnifying power is very considerably increased. A nosegay composed of a variety of delicate flowers, and even a single flower, such as the sea-pink, makes a splendid appearance in this way. A peacock’s feather, or even the fibres on a common quill, appear very beautiful, when placed in a proper light. The leaves of trees, particularly the leaf of the plane-tree, when placed against a window-pane, so that the light may shine through them—appear, in all their internal ramifications, more distinct, beautiful and interesting, than whenviewed in any other way; and in such views a large portion of the object is at once exhibited to the eye. In this case, the eye-piece of such a telescope as that alluded to requires to be drawn out 12 or 14 inches beyond the point of distinct vision for objects at a distance—and the distance between these near objects and the object-end of the telescope, is only about 3½ feet.
A telescope having a diagonal eye-piece presents a very pleasant view of near objects in this manner. With an instrument of this kind, I have frequently viewed the larger kind of small objects alluded to above, such as the leaves of shrubs and trees, flowers consisting of a variety of parts, the fibres of a peacock’s feather and similar objects. In this case the object-glass of the instrument, which is 10½ inches focal length, was brought within 22 inches of the object, and the eye looked down upon it, in the same manner, as when we view objects in a compound microscope. A common pocket achromatic telescope may be used for the purposes now stated, provided the tube in the eye-piece containing the two lenses next the object, be taken out, in which case the two glasses next the eye form an astronomical eye-piece, and the tubes may be drawn out 5 or 6 inches beyond the focal point for distant objects, and will produce distinct vision for objects not farther distant than about 20 or 24 inches. But, in this case, the objects to be viewed must be inverted, in order that they may be seen in their natural positions when viewed through the instrument. Telescopes of a large size and high magnifying powers may likewise be used with advantage for viewing very near objects in gardens adjacent to the room in which the instruments are placed, provided the sliding-tube next the eyehas a range of two or three inches beyond the point of vision for distant objects. In this case, a magnifying power of 100 times on a 3½ or a 5 feet achromatic produces a very pleasant effect. In making the observations to which I have now alluded, it is requisite in order to distinct vision, and to obtain a pleasing view of the objects, that the instrument should be placed on a pedestal, and capable of a motion in every direction. The adjustment for distinct vision may be made either by the sliding-tube, or by removing the telescope nearer to or farther from the object.
Light is one of the most wonderful and beneficial, and at the same time one of the most mysterious agents in the material creation. Though the sun from which it flows to this part of our system is nearly a hundred millions of miles from our globe, yet we perceive it as evidently, and feel its influence as powerfully, as if it emanated from no higher a region than the clouds. It supplies life and comfort to our physical system, and without its influence and operations on the various objects around us, we could scarcely subsist and participate of enjoyment for a single hour. It is diffused around us on every hand from its fountain the sun; and even the stars, though at a distance hundreds of thousands of times greater than that of the solar orb, transmit to our distant region a portion of this element. It gives beauty and fertility to the earth, it supports the vegetable and animal tribes, and is connected with the various motions which are going forwardthroughout the system of the universe. It unfolds to us the whole scenery of external nature—the lofty mountains and the expansive plains, the majestic rivers and the mighty ocean; the trees, the flowers, the crystal streams, and the vast canopy of the sky adorned with ten thousands of shining orbs. In short there is scarcely an object within the range of our contemplation, but what is exhibited to our understanding through the medium of light, or at least bears a certain relation to this enlivening and universal agent. When we consider the extreme minuteness of the rays of light, their inconceivable velocity, the invariable laws by which they act upon all bodies, the multifarious phenomena produced by their inflections, refractions and reflections, while their original properties remain the same; the endless variety of colours they produce on every part of our terrestrial creation, and the facility with which millions of rays pass through the smallest apertures, and pervade substances of great density, while every ray passes forward in the crowd without disturbing another, and produces its own specific impression—we cannot but regard this element as the most wonderful, astonishing and delightful part of the material creation. When we consider the admirable beauties and the exquisite pleasures of which light is the essential source, and how much its nature is still involved in mystery, notwithstanding the profound investigations of modern philosophers, we may well exclaim with the Poet:—