[1]Plate 1, fig. 3.
[1]Plate 1, fig. 3.
62.
Before, however, we proceed further, we have yet to observe some very remarkable cases of the vivacity with which the suggested colours appear in the neighbourhood of others: we allude to coloured shadows. To arrive at these we first turn our attention to shadows that are colourless or negative.
63.
A shadow cast by the sun, in its full brightness, on a white surface, gives us no impression of colour; it appears black, or, if a contrary light (here assumed to differ only in degree) can act upon it, it is only weaker, half-lighted, grey.
64.
Two conditions are necessary for the existence of coloured shadows: first, that the principal light tinge the white surface with some hue; secondly, that a contrary light illumine to a certain extent the cast shadow.
65.
Let a short, lighted candle be placed at twilight on a sheet of white paper. Between it and the declining daylight let a pencil be placedupright, so that its shadow thrown by the candle may be lighted, but not overcome, by the weak daylight: the shadow will appear of the most beautiful blue.
66.
That this shadow is blue is immediately evident; but we can only persuade ourselves by some attention that the white paper acts as a reddish yellow, by means of which the complemental blue is excited in the eye.—Note D.
67.
In all coloured shadows, therefore, we must presuppose a colour excited or suggested by the hue of the surface on which the shadow is thrown. This may be easily found to be the case by attentive consideration, but we may convince ourselves at once by the following experiment.
68.
Place two candles at night opposite each other on a white surface; hold a thin rod between them upright, so that two shadows be cast by it; take a coloured glass and hold it before one of the lights, so that the white paper appear coloured; at the same moment the shadow cast by the coloured light and slightly illumined by the colourless one will exhibit the complemental hue.
69.
An important consideration suggests itself here, to which we shall frequently have occasion to return. Colour itself is a degree of darknessσκιερόν; hence Kircher is perfectly right in calling itlumen opacatum. As it is allied to shadow, so it combines readily with it; it appears to us readily in and by means of shadow the moment a suggesting cause presents itself. We could not refrain from adverting at once to a fact which we propose to trace and develop hereafter.—Note E.
70.
Select the moment in twilight when the light of the sky is still powerful enough to cast a shadow which cannot be entirely effaced by the light of a candle. The candle may be so placed that a double shadow shall be visible, one from the candle towards the daylight, and another from the daylight towards the candle. If the former is blue the latter will appear orange-yellow: this orange-yellow is in fact, however, only the yellow-red light of the candle diffused over the whole paper, and whichbecomes visible in shadow.
71.
This is best exemplified by the former experiment with two candles and coloured glasses.
The surprising readiness with which shadow assumes a colour will again invite our attention in the further consideration of reflections and elsewhere.
72.
Thus the phenomena of coloured shadows may be traced to their cause without difficulty. Henceforth let any one who sees an instance of the kind observe only with what hue the light surface on which they are thrown is tinged. Nay, the colour of the shadow may be considered as a chromatoscope of the illumined surface, for the spectator may always assume the colour of the light to be the opposite of that of the shadow, and by an attentive examination may ascertain this to be the fact in every instance.
73.
These appearances have been a source of great perplexity to former observers: for, as they were remarked chiefly in the open air, where they commonly appeared blue, they were attributed to a certain inherent blue or blue colouring quality in the air. The inquirer can, however, convince himself, by the experiment with the candle in a room, that no kind of blue light or reflection is necessary to produce the effect in question. The experiment may be made on a cloudy day with white curtains drawnbefore the light, and in a room where no trace of blue exists, and the blue shadow will be only so much the more beautiful.
74.
De Saussure, in the description of his ascent of Mont Blanc, says, "A second remark, which may not be uninteresting, relates to the colour of the shadows. These, notwithstanding the most attentive observation, we never found dark blue, although this had been frequently the case in the plain. On the contrary, in fifty-nine instances we saw them once yellowish, six times pale bluish, eighteen times colourless or black, and thirty-four times pale violet. Some natural philosophers suppose that these colours arise from accidental vapours diffused in the air, which communicate their own hues to the shadows; not that the colours of the shadows are occasioned by the reflection of any given sky colour or interposition of any given air colour: the above observations seem to favour this opinion." The instances given by De Saussure may be now explained and classed with analogous examples without difficulty.
At a great elevation the sky was generally free from vapours, the sun shone in full force on the snow, so that it appeared perfectly white to the eye: in this case they saw the shadows quite colourless. If the air was charged with acertain degree of vapour, in consequence of which the light snow would assume a yellowish tone, the shadows were violet-coloured, and this effect, it appears, occurred oftenest. They saw also bluish shadows, but this happened less frequently; and that the blue and violet were pale was owing to the surrounding brightness, by which the strength of the shadows was mitigated. Once only they saw the shadow yellowish: in this case, as we have already seen (70), the shadow is cast by a colourless light, and slightly illumined by a coloured one.
75.
In travelling over the Harz in winter, I happened to descend from the Brocken towards evening; the wide slopes extending above and below me, the heath, every insulated tree and projecting rock, and all masses of both, were covered with snow or hoar-frost. The sun was sinking towards the Oder ponds[1]. During the day, owing to the yellowish hue of the snow, shadows tending to violet had already been observable; these might now be pronounced to be decidedly blue, as the illumined parts exhibited a yellow deepening to orange.
But as the sun at last was about to set, and its rays, greatly mitigated by the thicker vapours,began to diffuse a most beautiful red colour over the whole scene around me, the shadow colour changed to a green, in lightness to be compared to a sea-green, in beauty to the green of the emerald. The appearance became more and more vivid: one might have imagined oneself in a fairy world, for every object had clothed itself in the two vivid and so beautifully harmonising colours, till at last, as the sun went down, the magnificent spectacle was lost in a grey twilight, and by degrees in a clear moon-and-starlight night.
76.
One of the most beautiful instances of coloured shadows may be observed during the full moon. The candle-light and moon-light may be contrived to be exactly equal in force; both shadows may be exhibited with equal strength and clearness, so that both colours balance each other perfectly. A white surface being placed opposite the full moon, and the candle being placed a little on one side at a due distance, an opaque body is held before the white plane, A double shadow will then be seen: that cast by the moon and illumined by the candle-light will be a powerful red-yellow; and contrariwise, that cast by the candle and illumined by the moon will appear of the most beautiful blue. The shadow, composed of the union of the two shadows, wherethey cross each other, is black. The yellow shadow (74) cannot perhaps be exhibited in a more striking manner. The immediate vicinity of the blue and the interposing black shadow make the appearance the more agreeable. It will even be found, if the eye dwells long on these colours, that they mutually evoke and enhance each other, the increasing red in the one still producing its contrast, viz. a kind of sea-green.
77.
We are here led to remark that in this, and in all cases, a moment or two may perhaps be necessary to produce the complemental colour. The retina must be first thoroughly impressed with the demanding hue before the responding one can be distinctly observable.
78.
When divers are under water, and the sunlight shines into the diving-bell, everything is seen in a red light (the cause of which will be explained hereafter), while the shadows appear green. The very same phenomenon which I observed on a high mountain (75) is presented to others in the depths of the sea, and thus Nature throughout is in harmony with herself.
79.
Some observations and experiments which equally illustrate what has been stated with regardto coloured objects and coloured shadows may be here added. Let a white paper blind be fastened inside the window on a winter evening; in this blind let there be an opening, through which the snow of some neighbouring roof can be seen. Towards dusk let a candle be brought into the room; the snow seen through the opening will then appear perfectly blue, because the paper is tinged with warm yellow by the candle-light. The snow seen through the aperture is here equivalent to a shadow illumined by a contrary light (76), and may also represent a grey disk on a coloured surface (56).
80.
Another very interesting experiment may conclude these examples. If we take a piece of green glass of some thickness, and hold it so that the window bars be reflected in it, they will appear double owing to the thickness of the glass. The image which is reflected from the under surface of the glass will be green; the image which is reflected from the upper surface, and which should be colourless, will appear red.
The experiment may be very satisfactorily made by pouring water into a vessel, the inner surface of which can act as a mirror; for both reflections may first be seen colourless while the water is pure, and then by tinging it, they will exhibit two opposite hues.
[1]Reservoirs in which water is collected from various small streams, to work the mines.—T.
[1]Reservoirs in which water is collected from various small streams, to work the mines.—T.
81.
Light, in its full force, appears purely white, and it gives this impression also in its highest degree of dazzling splendour. Light, which is not so powerful, can also, under various conditions, remain colourless. Several naturalists and mathematicians have endeavoured to measure its degrees—Lambert, Bouguer, Rumford.
82.
Yet an appearance of colour presently manifests itself in fainter lights, for in their relation to absolute light they resemble the coloured spectra of dazzling objects (39).
83.
A light of any kind becomes weaker, either when its own force, from whatever cause, is diminished, or when the eye is so circumstanced or placed, that it cannot be sufficiently impressed by the action of the light. Those appearances which may be called objective, come under the head of physical colours. We will only advert here to the transition from white to red heat in glowing iron. We may also observethat the flames of lights at night appear redder in proportion to their distance from the eye.—Note F.
84.
Candle-light at night acts as yellow when seen near; we can perceive this by the effect it produces on other colours. At night a pale yellow is hardly to be distinguished from white; blue approaches to green, and rose-colour to orange.
85.
Candle-light at twilight acts powerfully as a yellow light: this is best proved by the purple blue shadows which, under these circumstances, are evoked by the eye.
86.
The retina may be so excited by a strong light that it cannot perceive fainter lights (11): if it perceive these they appear coloured: hence candle-light by day appears reddish, thus resembling, in its relation to fuller light, the spectrum of a dazzling object; nay, if at night we look long and intently on the flame of a light, it appears to increase in redness.
87.
There are faint lights which, notwithstanding their moderate lustre, give an impression of awhite, or, at the most, of a light yellow appearance on the retina; such as the moon in its full splendour. Rotten wood has even a kind of bluish light. All this will hereafter be the subject of further remarks.
88.
If at night we place a light near a white or greyish wall so that the surface be illumined from this central point to some extent, we find, on observing the spreading light at some distance, that the boundary of the illumined surface appears to be surrounded with a yellow circle, which on the outside tends to red-yellow. We thus observe that when light direct or reflected does not act in its full force, it gives an impression of yellow, of reddish, and lastly even of red. Here we find the transition to halos which we are accustomed to see in some mode or other round luminous points.
89.
Halos may be divided into subjective and objective. The latter will be considered under the physical colours; the first only belong here. These are distinguished from the objectivehalos by the circumstance of their vanishing when the point of light which produces them on the retina is covered.
90.
We have before noticed the impression of a luminous object on the retina, and seen that it appears larger: but the effect is not at an end here, it is not confined to the impression of the image; an expansive action also takes place, spreading from the centre.
91.
That a nimbus of this kind is produced round the luminous image in the eye may be best seen in a dark room, if we look towards a moderately large opening in the window-shutter. In this case the bright image is surrounded by a circular misty light. I saw such a halo bounded by a yellow and yellow-red circle on opening my eyes at dawn, on an occasion when I passed several nights in a bed-carriage.
92.
Halos appear most vivid when the eye is susceptible from having been in a state of repose. A dark background also heightens their appearance. Both causes account for our seeing them so strong if a light is presented to the eyeson waking at night. These conditions were combined when Descartes after sleeping, as he sat in a ship, remarked such a vividly-coloured halo round the light.
93.
A light must shine moderately, not dazzle, in order to produce the impression of a halo in the eye; at all events the halos of dazzling lights cannot be observed. We see a splendour of this kind round the image of the sun reflected from the surface of water.
94.
A halo of this description, attentively observed, is found to be encircled towards its edge with a yellow border: but even here the expansive action, before alluded to, is not at an end, but appears still to extend in varied circles.
95.
Several cases seem to indicate a circular action of the retina, whether owing to the round form of the eye itself and its different parts, or to some other cause.
96.
If the eye is pressed only in a slight degree from the inner corner, darker or lighter circlesappear. At night, even without pressure, we can sometimes perceive a succession of such circles emerging from, or spreading over, each other.
97.
We have already seen that a yellow border is apparent round the white space illumined by a light placed near it. This may be a kind of objective halo. (88.)
98.
Subjective halos may be considered as the result of a conflict between the light and a living surface. From the conflict between the exciting principle and the excited, an undulating motion arises, which may be illustrated by a comparison with the circles on water. The stone thrown in drives the water in all directions; the effect attains a maximum, it reacts, and being opposed, continues under the surface. The effect goes on, culminates again, and thus the circles are repeated. If we have ever remarked the concentric rings which appear in a glass of water on trying to produce a tone by rubbing the edge; if we call to mind the intermitting pulsations in the reverberations of bells, we shall approach a conception of what may take place on the retina when the image of a luminous object impinges on it, not to mentionthat as a living and elastic structure, it has already a circular principle in its organisation.—Note G.
99.
The bright circular space which appears round the shining object is yellow, ending in red: then follows a greenish circle, which is terminated by a red border. This appears to be the usual phenomenon where the luminous body is somewhat considerable in size. These halos become greater the more distant we are from the luminous object.
100.
Halos may, however, appear extremely small and numerous when the impinging image is minute, yet powerful, in its effect. The experiment is best made with a piece of gold-leaf placed on the ground and illumined by the sun. In these cases the halos appear in variegated rays. The iridescent appearance produced in the eye when the sun pierces through the leaves of trees seems also to belong to the same class of phenomena.
101.
We are now sufficiently acquainted with the physiological colours to distinguish them from the pathological. We know what appearances belong to the eye in a healthy state, and are necessary to enable the organ to exert its complete vitality and activity.
102.
Morbid phenomena indicate in like manner the existence of organic and physical laws: for if a living being deviates from those rules with reference to which it is constructed, it still seeks to agree with the general vitality of nature in conformity with general laws, and throughout its whole course still proves the constancy of those principles on which the universe has existed, and by which it is held together.
103.
We will here first advert to a very remarkable state in which the vision of many persons is found to be. As it presents a deviation from the ordinary mode of seeing colours, it might be fairly classed under morbid impressions; but as it is consistent in itself, as it often occurs,may extend to several members of a family, and probably does not admit of cure, we may consider it as bordering only on the nosological cases, and therefore place it first.
104.
I was acquainted with two individuals not more than twenty years of age, who were thus affected: both had bluish-grey eyes, an acute sight for near and distant objects, by day-light and candle-light, and their mode of seeing colours was in the main quite similar.
105.
They agreed with the rest of the world in denominating white, black, and grey in the usual manner. Both saw white untinged with any hue. One saw a somewhat brownish appearance in black, and in grey a somewhat reddish tinge. In general they appeared to have a very delicate perception of the gradations of light and dark.
106.
They appeared to see yellow, red-yellow, and yellow-red,[1]like others: in the last case they said they saw the yellow passing as it were over the red as if glazed: some thickly-ground carmine, which had dried in a saucer, they called red.
107.
But now a striking difference presented itself. If the carmine was passed thinly over the white saucer, they would compare the light colour thus produced to the colour of the sky, and call it blue. If a rose was shown them beside it, they would, in like manner, call it blue; and in all the trials which were made, it appeared that they could not distinguish light blue from rose-colour. They confounded rose-colour, blue, and violet on all occasions: these colours only appeared to them to be distinguished from each other by delicate shades of lighter, darker, intenser, or fainter appearance.
108.
Again they could not distinguish green from dark orange, nor, more especially, from a red brown.
109.
If any one, accidentally conversing with these individuals, happened to question them about surrounding objects, their answers occasioned the greatest perplexity, and the interrogator began to fancy his own wits were out of order. With some method we may, however, approach to a nearer knowledge of the law of this deviation from the general law.
110.
These persons, as may be gathered from what has been stated, saw fewer colours than other people: hence arose the confusion of different colours. They called the sky rose-colour, and the rose blue, orvice versâ. The question now is: did they see both blue or both rose-colour? did they see green orange, or orange green?
111.
This singular enigma appears to solve itself, if we assume that they saw no blue, but, instead of it, a light pure red, a rose-colour. We can comprehend what would be the result of this by means of the chromatic diagram.
112.
If we take away blue from the chromatic circle we shall miss violet and green as well. Pure red occupies the place of blue and violet, and in again mixing with yellow the red produces orange where green should be.
113.
Professing to be satisfied with this mode of explanation, we have named this remarkable deviation from ordinary vision "Acyanoblepsia."[2]We have prepared some coloured figures forits further elucidation, and in explaining these we shall add some further details. Among the examples will be found a landscape, coloured in the mode in which the individuals alluded to appeared to see nature: the sky rose-colour, and all that should be green varying from yellow to brown red, nearly as foliage appears to us in autumn[3].—Note H.
114.
We now proceed to speak of morbid and other extraordinary affections of the retina, by which the eye may be susceptible of an appearance of light without external light, reserving for a future occasion the consideration of galvanic light.
115.
If the eye receives a blow, sparks seem to spread from it. In some states of body, again, when the blood is heated, and the system much excited, if the eye is pressed first gently, and then more and more strongly, a dazzling and intolerable light may be excited.
116.
If those who have been recently couched experience pain and heat in the eye, they frequentlysee fiery flashes and sparks: these symptoms last sometimes for a week or fortnight, or till the pain and heat diminish.
117.
A person suffering from ear-ache saw sparks and balls of light in the eye during each attack, as long as the pain lasted.
118.
Persons suffering from worms often experience extraordinary appearances in the eye, sometimes sparks of fire, sometimes spectres of light, sometimes frightful figures, which they cannot by an effort of the will cease to see: sometimes these appearances are double.
119.
Hypochondriacs frequently see dark objects, such as threads, hairs, spiders, flies, wasps. These appearances also exhibit themselves in the incipient hard cataract. Many see semi-transparent small tubes, forms like wings of insects, bubbles of water of various sizes, which fall slowly down, if the eye is raised: sometimes these congregate together so as to resemble the spawn of frogs; sometimes they appear as complete spheres, sometimes in the form of lenses.
120.
As light appeared, in the former instances,without external light, so also these images appear without corresponding external objects. The images are sometimes transient, sometimes they last during the patient's life. Colour, again, frequently accompanies these impressions: for hypochondriacs often see yellow-red stripes in the eye: these are generally more vivid and numerous in the morning, or when lasting.
121.
We have before seen that the impression of any object may remain for a time in the eye: this we have found to be a physiological phenomenon (23): the excessive duration of such an impression, on the other band, may be considered as morbid.
122.
The weaker the organ the longer the impression of the image lasts. The retina does not so soon recover itself; and the effect may be considered as a kind of paralysis (28).
123.
This is not to be wondered at in the case of dazzling lights. If any one looks at the sun, he may retain the image in his eyes for several days. Boyle relates an instance of ten years.
124.
The same takes place, in a certain degree, withregard to objects that are not dazzling. Büsch relates of himself that the image of an engraving, complete in all its parts, was impressed on his eye for seventeen minutes.
125.
A person inclined to fulness of blood retained the image of a bright red calico, with white spots, many minutes in the eye, and saw it float before everything like a veil. It only disappeared by rubbing the eye for some time.
126.
Scherfer observes that the red colour, which is the consequence of a powerful impression of light, may last for some hours.
127.
As we can produce an appearance of light on the retina by pressure on the eyeball, so by a gentle pressure a red colour appears, thus corresponding with the after-image of an impression of light.
128.
Many sick persons, on awaking, see everything in the colour of the morning sky, as if through a red veil: so, if in the evening they doze and wake again, the same appearance presents itself. It remains for some minutes, andalways disappears if the eye is rubbed a little. Red stars and balls sometimes accompany the impression. This state may last for a considerable time.
129.
The aëronauts, particularly Zambeccari and his companions, relate that they saw the moon blood-red at the highest elevation. As they had ascended above the vapours of the earth, through which we see the moon and sun naturally of such a colour, it may be suspected that this appearance may be classed with the pathological colours. The senses, namely, may be so influenced by an unusual state, that the whole nervous system, and particularly the retina, may sink into a kind of inertness and inexcitability. Hence it is not impossible that the moon might act as a very subdued light, and thus produce the impression of the red colour. The sun even appeared blood-red to the aëronauts of Hamburgh.
If those who are at some elevation in a balloon scarcely hear each other speak, may not this, too, be attributed to the inexcitable state of the nerves as well as to the thinness of the air?
130.
Objects are often seen by sick persons in variegated colours. Boyle relates an instanceof a lady, who, after a fall by which an eye was bruised, saw all objects, but especially white objects, glittering in colours, even to an intolerable degree.
131.
Physicians give the name of "Chrupsia" to an affection of the sight, occurring in typhoid maladies. In these cases the patients state that they see the boundaries of objects coloured where light and dark meet. A change probably takes place in the humours of the eye, through which their achromatism is affected.
132.
In cases of milky cataract, a very turbid crystalline lens causes the patient to see a red light. In a case of this kind, which was treated by the application of electricity, the red light changed by degrees to yellow, and at last to white, when the patient again began to distinguish objects. These changes of themselves warranted the conclusion that the turbid state of the lens was gradually approaching the transparent state. We shall be enabled easily to trace this effect to its source as soon as we become better acquainted with the physical colours.
133.
If again it may be assumed that a jaundicedpatient sees through an actually yellow-coloured humour, we are at once referred to the department of chemical colours, and it is thus evident that we can only thoroughly investigate the chapter of pathological colours when we have made ourselves acquainted with the whole range of the remaining phenomena. What has been adduced may therefore suffice for the present, till we resume the further consideration of this portion of our subject.
134.
In conclusion we may, however, at once advert to some peculiar states or dispositions of the organ.
There are painters who, instead of rendering the colours of nature, diffuse a general tone, a warm or cold hue, over the picture. In some, again, a predilection for certain colours displays itself; in others a want of feeling for harmony.
135.
Lastly, it is also worthy of remark, that savage nations, uneducated people, and children have a great predilection for vivid colours; that animals are excited to rage by certain colours; that people of refinement avoid vivid colours in their dress and the objects that are about them, and seem inclined to banish them altogether from their presence.—Note I.
[1]It has been found necessary to follow the author's nomenclature throughout—T.
[1]It has been found necessary to follow the author's nomenclature throughout—T.
[2]Non-perception of blue.
[2]Non-perception of blue.
[3]It has not been thought necessary to copy the plates here referred to.—T.
[3]It has not been thought necessary to copy the plates here referred to.—T.
136.
We give this designation to colours which are produced by certain material mediums: these mediums, however, have no colour themselves, and may be either transparent, semi-transparent yet transmitting light, or altogether opaque. The colours in question are thus produced in the eye through such external given causes, or are merely reflected to the eye when by whatever means they are already produced without us. Although we thus ascribe to them a certain objective character, their distinctive quality still consists in their being transient, and not to be arrested.
137.
They are called by former investigatorscolores apparentes, fluxi, fugitivi, phantastici, falsi, variantes. They are also calledspeciosiandemphatici, on account of their striking splendour. They are immediately connected with the physiological colours, and appear to have but little more reality: for, while in the productionof the physiological colours the eye itself was chiefly efficient, and we could only perceive the phenomena thus evoked within ourselves, but not without us, we have now to consider the fact that colours are produced in the eye by means of colourless objects; that we thus too have a colourless surface before us which is acted upon as the retina itself is, and that we can perceive the appearance produced upon it without us. In such a process, however, every observation will convince us that we have to do with colours in a progressive and mutable, but not in a final or complete, state.
138.
Hence, in directing our attention to these physical colours, we find it quite possible to place an objective phenomenon beside a subjective one, and often by means of the union of the two successfully to penetrate farther into the nature of the appearance.
139.
Thus, in the observations by which we become acquainted with the physical colours, the eye is not to be considered as acting alone; nor is the light ever to be considered in immediate relation with the eye: but we direct our attention especially to the various effects produced by mediums, those mediums being themselves colourless.
140.
Light under these circumstances may be affected by three conditions. First, when it flashes back from the surface of a medium; in considering whichcatoptricalexperiments invite our attention. Secondly, when it passes by the edge of a medium: the phenomena thus produced were formerly calledperioptical; we prefer the termparoptical. Thirdly, when it passes through either a merely light-transmitting or an actually transparent body; thus constituting a class of appearances on whichdioptricalexperiments are founded. We have called a fourth class of physical coloursepoptical, as the phenomena exhibit themselves on the colourless surface of bodies under various conditions, without previous or actual dye (βαφή).—Note K.
141.
In examining these categories with reference to our three leading divisions, according to which we consider the phenomena of colours in a physiological, physical, or chemical view, we find that the catoptrical colours are closely connected with the physiological; the paroptical are already somewhat more distinct and independent; the dioptrical exhibit themselves as entirely and strictly physical, and as having a decidedly objective character; the epoptical, although still only apparent, may be considered as the transition to the chemical colours.
142.
If we were desirous of prosecuting our investigation strictly in the order of nature, we ought to proceed according to the classification which has just been made; but in didactic treatises it is not of so much consequence to connect as to duly distinguish the various divisions of a subject, in order that at last, when every single class and case has been presented to the mind, the whole may be embraced in one comprehensive view. We therefore turn our attention forthwith to the dioptrical class, in order at once to give the reader the full impression of the physical colours, and to exhibit their characteristics the more strikingly.
143.
Colours are called dioptrical when a colourless medium is necessary to produce them; the medium must be such that light and darkness can act through it either on the eye or on opposite surfaces. It is thus required that the medium should be transparent, or at least capable, to a certain degree, of transmitting light.
144.
According to these conditions we divide the dioptrical phenomena into two classes, placing in the first those which are produced by means of imperfectly transparent, yet light-transmitting mediums; and in the second such as are exhibited when the medium is in the highest degree transparent.
145.
Space, if we assume it to be empty, would have the quality of absolute transparency to our vision. If this space is filled so that the eye cannot perceive that it is so, there exists a more or less material transparent medium, which may be of the nature of air and gas, may be fluid or even solid.
146.
The pure and light-transmitting semi-transparent medium is only an accumulated form of the transparent medium. It may therefore be presented to us in three modes.
147.
The extreme degree of this accumulation is white; the simplest, brightest, first, opaque occupation of space.
148.
Transparency itself, empirically considered, is already the first degree of the opposite state. The intermediate degrees from this point to opaque white are infinite.
149.
At whatever point short of opacity we arrest the thickening medium, it exhibits simple and remarkable phenomena when placed in relation with light and darkness.
150.
The highest degree of light, such as that of the sun, of phosphorus burning in oxygen, is dazzling and colourless: so the light of the fixed stars is for the most part colourless. This light, however, seen through a medium but very slightly thickened, appears to us yellow. If the density of such a medium be increased, or if its volume become greater, we shall see the light gradually assume a yellow-red hue, which at last deepens to a ruby-colour.—Note L.
151.
If on the other hand darkness is seen through a semi-transparent medium, which is itself illumined by a light striking on it, a blue colour appears: this becomes lighter and paler as the density of the medium is increased, but on the contrary appears darker and deeper the more transparent the medium becomes: in the least degree of dimness short of absolute transparence, always supposing a perfectly colourless medium, this deep blue approaches the most beautiful violet.
152.
If this effect takes place in the eye as here described, and may thus be pronounced to be subjective, it remains further to convince ourselves of this by objective phenomena. For a light thus mitigated and subdued illumines all objects in like manner with a yellow, yellow-red, or red hue; and, although the effect of darkness through the non-transparent medium does not exhibit itself so powerfully, yet the blue sky displays itself in the camera obscura very distinctly on white paper, as well as every other material colour.
153.
In examining the cases in which this importantleading phenomenon appears, we naturally mention the atmospheric colours first: most of these may be here introduced in order.
154.
The sun seen through a certain degree of vapour appears with a yellow disk; the centre is often dazzlingly yellow when the edges are already red. The orb seen through a thick yellow mist appears ruby-red (as was the case in 1794, even in the north); the same appearance is still more decided, owing to the state of the atmosphere, when the scirocco prevails in southern climates: the clouds generally surrounding the sun in the latter case are of the same colour, which is reflected again on all objects.
The red hues of morning and evening are owing to the same cause. The sun is announced by a red light, in shining through a greater mass of vapours. The higher he rises, the yellower and brighter the light becomes.
155.
If the darkness of infinite space is seen through atmospheric vapours illumined by the day-light, the blue colour appears. On high mountains the sky appears by day intensely blue, owing to the few thin vapours that float before the endless dark space: as soon as we descend in thevalleys, the blue becomes lighter; till at last, in certain regions, and in consequence of increasing vapours, it altogether changes to a very pale blue.
156.
The mountains, in like manner, appear to us blue; for, as we see them at so great a distance that we no longer distinguish the local tints, and as no light reflected from their surface acts on our vision, they are equivalent to mere dark objects, which, owing to the interposed vapours, appear blue.
157.
So we find the shadowed parts of nearer objects are blue when the air is charged with thin vapours.
158.
The snow-mountains, on the other hand, at a great distance, still appear white, or approaching to a yellowish hue, because they act on our eyes as brightness seen through atmospheric vapour.
159.
The blue appearance at the lower part of the flame of a candle belongs to the same class of phenomena. If the flame be held before a white ground, no blue will be seen, but this colour will immediately appear if the flame is opposedto a black ground. This phenomenon may be exhibited most strikingly with a spoonful of lighted spirits of wine. We may thus consider the lower part of the flame as equivalent to the vapour which, although infinitely thin, is still apparent before the dark surface; it is so thin, that one may easily see to read through it: on the other hand, the point of the flame which conceals objects from our sight is to be considered as a self-illuminating body.
160.
Lastly, smoke is also to be considered as a semi-transparent medium, which appears to us yellow or reddish before a light ground, but blue before a dark one.
161.
If we now turn our attention to fluid mediums, we find that water, deprived in a very slight degree of its transparency, produces the same effects.
162.
The infusion of the lignum nephriticum (guilandina Linnæi), which formerly excited so much attention, is only a semi-transparent liquor, which in dark wooden cups must appear blue, but held towards the sun in a transparent glass must exhibit a yellow appearance.
163.
A drop of scented water, of spirit varnish, of several metallic solutions, may be employed to give various degrees of opacity to water for such experiments. Spirit of soap perhaps answers best.
164.
The bottom of the sea appears to divers of a red colour in bright sunshine: in this case the water, owing to its depth, acts as a semi-transparent medium. Under these circumstances, they find the shadows green, which is the complemental colour.
165.
Among solid mediums the opal attracts our attention first: its colours are, at least, partly to be explained by the circumstance that it is, in fact, a semi-transparent medium, through which sometimes light, sometimes dark, substrata are visible.
166.
For these experiments, however, the opal-glass (vitrum astroides, girasole) is the most desirable material. It is prepared in various ways, and its semi-opacity is produced by metallic oxydes. The same effect is produced also by melting pulverised and calcined bones togetherwith the glass, on which account it is also known by the name ofbeinglas; but, prepared in this mode, it easily becomes too opaque.
167.
This glass may be adapted for experiments in various ways: it may either be made in a very slight degree non-transparent, in which case the light seen through various layers placed one upon the other may be deepened from the lightest yellow to the deepest red, or, if made originally more opaque, it may be employed in thinner or thicker laminæ. The experiments may be successfully made in both ways: in order, however, to see the bright blue colour, the glass should neither be too opaque nor too thick. For, as it is quite natural that darkness must act weakly through the semi-transparent medium, so this medium, if too thick, soon approaches whiteness.
168.
Panes of glass throw a yellow light on objects through those parts where they happen to be semi-opaque, and these same parts appear blue if we look at a dark object through them.
169.
Smoked glass may be also mentioned here, and is, in like manner, to be considered as a semi-opaque medium. It exhibits the sun moreor less ruby-coloured; and, although this appearance may be attributed to the black-brown colour of the soot, we may still convince ourselves that a semi-transparent medium here acts if we hold such a glass moderately smoked, and lit by the sun on the unsmoked side, before a dark object, for we shall then perceive a bluish appearance.
170.
A striking experiment may be made in a dark room with sheets of parchment. If we fasten a piece of parchment before the opening in the window-shutter when the sun shines, it will appear nearly white; by adding a second, a yellowish colour appears, which still increases as more leaves are added, till at last it changes to red.
171.
A similar effect, owing to the state of the crystalline lens in milky cataract, has been already adverted to (131).
172.
Having now, in tracing these phenomena, arrived at the effect of a degree of opacity scarcely capable of transmitting light, we may here mention a singular appearance which was owing to a momentary state of this kind.
A portrait of a celebrated theologian had been painted some years before the circumstance to which we allude, by an artist who was known to have considerable skill in the management of his materials. The very reverend individual was represented in a rich velvet dress, which was not a little admired, and which attracted the eye of the spectator almost more than the face. The picture, however, from the effect of the smoke of lamps and dust, had lost much of its original vivacity. It was, therefore, placed in the hands of a painter, who was to clean it, and give it a fresh coat of varnish. This person began his operations by carefully washing the picture with a sponge: no sooner, however, had he gone over the surface once or twice, and wiped away the first dirt, than to his amazement the black velvet dress changed suddenly to a light blue plush, which gave the ecclesiastic a very secular, though somewhat old-fashioned, appearance. The painter did not venture to go on with his washing: he could not comprehend how a light blue should be the ground of the deepest black, still less how he could so suddenly have removed a glazing colour capable of converting the one tint to the other.
At all events, he was not a little disconcerted at having spoilt the picture to such an extent. Nothing to characterize the ecclesiastic remainedbut the richly-curled round wig, which made the exchange of a faded plush for a handsome new velvet dress far from desirable. Meanwhile, the mischief appeared irreparable, and the good artist, having turned the picture to the wall, retired to rest with a mind ill at ease. But what was his joy the next morning, when, on examining the picture, he beheld the black velvet dress again in its full splendour. He could not refrain from again wetting a corner, upon which the blue colour again appeared, and after a time vanished. On hearing of this phenomenon, I went at once to see the miraculous picture. A wet sponge was passed over it in my presence, and the change quickly took place. I saw a somewhat faded, but decidedly light blue plush dress, the folds under the arm being indicated by some brown strokes.
I explained this appearance to myself by the doctrine of the semi-opaque medium. The painter, in order to give additional depth to his black, may have passed some particular varnish over it: on being washed, this varnish imbibed some moisture, and hence became semi-opaque, in consequence of which the black underneath immediately appeared blue. Perhaps those who are practically acquainted with the effect of varnishes may, through accident or contrivance, arrive at some means of exhibiting this singular appearance, as an experiment, to thosewho are fond of investigating natural phenomena. Notwithstanding many attempts, I could not myself succeed in re-producing it.
173.
Having now traced the most splendid instances of atmospheric appearances, as well as other less striking yet sufficiently remarkable cases, to the leading examples of semi-transparent mediums, we have no doubt that attentive observers of nature will carry such researches further, and accustom themselves to trace and explain the various appearances which present themselves in every-day experience on the same principle: we may also hope that such investigators will provide themselves with an adequate apparatus in order to place remarkable facts before the eyes of others who may be desirous of information.
174.
We venture, once for all, to call the leading appearance in question, as generally described in the foregoing pages, a primordial and elementary phenomenon; and we may here be permitted at once to state what we understand by the term.
175.
The circumstances which come under our notice in ordinary observation are, for the most part, insulated cases, which, with some attention, admitof being classed under general leading facts. These again range themselves under theoretical rubrics which are more comprehensive, and through which we become better acquainted with certain indispensable conditions of appearances in detail. From henceforth everything is gradually arranged under higher rules and laws, which, however, are not to be made intelligible by words and hypotheses to the understanding merely, but, at the same time, by real phenomena to the senses. We call these primordial phenomena, because nothing appreciable by the senses lies beyond them, on the contrary, they are perfectly fit to be considered as a fixed point to which we first ascended, step by step, and from which we may, in like manner, descend to the commonest case of every-day experience. Such an original phenomenon is that which has lately engaged our attention. We see on the one side light, brightness; on the other darkness, obscurity: we bring the semi-transparent medium between the two, and from these contrasts and this medium the colours develop themselves, contrasted, in like manner, but soon, through a reciprocal relation, directly tending again to a point of union.[1]
176.
With this conviction we look upon the mistake that has been committed in the investigation of this subject to be a very serious one, inasmuch as a secondary phenomenon has been thus placed higher in order—the primordial phenomenon has been degraded to an inferior place; nay, the secondary phenomenon has been placed at the head, a compound effect has been treated as simple, a simple appearance as compound: owing to this contradiction, the most capricious complication and perplexity have been introduced into physical inquiries, the effects of which are still apparent.
177.
But when even such a primordial phenomenon is arrived at, the evil still is that we refuse to recognise it as such, that we still aim at something beyond, although it would become us to confess that we are arrived at the limits of experimental knowledge. Let the observer of nature suffer the primordial phenomenon to remain undisturbed in its beauty; let the philosopher admit it into his department, and he will find that important elementary facts are a worthier basis for further operations than insulated cases, opinions, and hypotheses.—Note M.