by Grant Allen
The reasons for this difference must surely be very striking ones. It seems curious that such similar agencies should be so diversely cognised, or should escape our cognisance altogether. And it is for the purpose of bringing into clear relief so strange a fact that I have chosen what doubtless seemed at first sight an awkward and unfamiliar mode of envisaging a well-known subject. The question why we have two distinct methods for perceiving two closely allied forms of æther-waves, and no method at all for perceiving the third, is a question which evolutionism is bound to answer before it proceeds to the minor discrimination of those lesser differences known as colours.
For when we look at the matter objectively, we see at once that each colour differs from its neighbour in just the same manner as heat differs from light, though only to a less degree. Accordingly, we must ask first, Why are the senses of animals so differently affected by the extremes and the mean of the solar undulations? And when we have answered that question we may go on to the next, How did the various minor undulations of mean rapidity come to have differential sensations attached to them in consciousness?
Fortunately, the answer is not a very difficult one. The slower and more massive undulations, which we know as heat-waves, produce very marked results even upon inorganic bodies, while their effects upon organic matter are obvious and enormously important. To the animal, cold is death and warmth is life. Hence it is not astonishing that animals should very early have developed a sense which informed them of the changes of temperature taking place in their vicinity; and that this sense should have been equally diffused over the whole organism. Æther-waves of slow vibration are capable of setting up motion in the molecules of all bodies upon which they impinge, as we know familiarly when we touch a stone on the summer beach, or grasp a poker which has lain long in front of the fire; and the motion so absorbed we call warmth: while, on the other hand, molecules in rapid motion give up their energy to the surrounding æther, as we also know when a red-hot poker cools, or when we expose our faces to the chilly wind of winter; and the loss of motion so induced we call cold. In either case, the immediate effects are so highly important to animal life, that we may well imagine the accompanying sensations to be amongst the earliest which evolution could have produced. As soon as moving creatures began to feel at all, they probably began to feel heat and cold.
The æther-waves of middle frequency, however, do not produce such plain and universal results. If we interpose a slab of rock-salt in the course of a solar beam, we can sift out of it all the slower undulations (or heat-waves), which are selected and absorbed by the salt itself. On placing our hands in the path of the remaining wavelets, we do not experience any feeling of heat whatsoever. And if we put a piece of inorganic matter — say a pebble — in the course of the sifted ray, we shall find that it is similarly unaffected in temperature or structure. The thermopile conclusively shows us that little or no immediate mechanical power is left in the wavelets which pass through the rock-salt. If we examine the results which these middle undulations produce upon the world at large, we shall arrive at similar conclusions. While to the heat-waves are due the conspicuous differences of summer and winter, ice, snow, and rain, the poles and the tropics, besides the great phenomena of ocean-currents, winds, evaporation, clouds, rainfall, and atmospheric disturbances generally; their companions, the light-waves, scarcely produce any noticeable effects at all. Falling upon the mass of the earth’s surface, they are not, like the slower undulations, absorbed and communicated through the substance on which they impinge, but are reflected and twisted back upon space in every possible direction. Even if they are partially taken in by the matter on which they fall, yet the greater portion of them are returned without effecting any change in its arrangement; and if, as in the case of what we call a black surface, a large number or the whole of them are absorbed and retained, they are yet degraded by the process into the form of heat-waves, from which they cannot be consciously discriminated except by indirect means. These middle waves could not, therefore, prove of any great importance to animal life in its earliest days; and we need not wonder that no sense for their perception was at first developed.
There is one conspicuous exception, however, to this comparative inertness of the light-waves — I mean the case of plants. In their leaves, the middle and quickest ætherial undulations become the agents for effecting great chemical and physical changes, upon which the whole course of mundane life entirely depends. But these facts, all-important in themselves, do not directly affect our present question. Light is essential to animal life, because it is essential to the plants upon which, mediately or immediately, animal life subsists. But a perception or discrimination of light is not at all necessary, except in a very roundabout and derivative way. Why it has arisen at all we may next briefly inquire.
The light-waves falling upon a body do not largely affect it, as a rule, in any way. They may occasionally be employed in bringing about slight changes of its superficial molecules, but they do not penetrate deeply or work conspicuous rearrangements of its whole substance. Nevertheless, the power of discriminating them may indirectly benefit an animal organism. If a jelly-fish, swimming at the water’s top, has eyelets upon which the incident light-waves produce distinct effects, it may be warned of the approaching enemy, or informed of passing prey, by having the path of the æther-waves cut off from above. Still more valuable will the nascent sense become, if, instead of being restricted to the full force of directly incident undulations, it is capable of being impressed by reflected waves. In this case, not only will the creature be conscious of objects passing between it and the source of light, but it will be able to receive varying stimulations from all surrounding objects upon which the light falls. The more highly developed its sight becomes (for we may now use the language of ordinary life without fear of ambiguity), the more clearly will it be affected by the beams which are twisted about and returned upon space from every neighbouring body. Until at last that very fact in the light-waves which made them originally so unimportant — the fact that they glance off every object they hit like a ball rebounding from a wall — gives them, in our eyes, the greatest value, by enabling us to discriminate from a distance the shape and texture of all we see, without the trouble of actual examination by the hands and fingers.
But this specialised sense is hardly likely to spread itself over the whole body, like the sense of heat and cold. Not only should we derive no advantage from being all eye, but we should be positively incommoded rather than benefited by such an arrangement. It will only be in certain special spots or ocelli that the perception of light will probably begin; and as the sense strengthens, we shall find these spots becoming fewer and fewer, until in the approximately perfect organisms they are reduced to the two conspicuous orbs which we commonly call eyes. All such questions, however, must be left over for a while, until we come to examine the development of the rudimentary vision. At present we must hurry on to reach our proper subject — the objective nature of colour.
As for the third class of ætherial undulations, the quickest or chemical waves, their effects are so slight and inconspicuous that we have never had occasion to develop any sense whatsoever for their perception. It is only quite recently, and by quite indirect methods (chiefly through the investigations of the earliest photographers), that we have come to recognise their existence at all. Neither upon inorganic substances nor upon animal bodies do they produce any striking result; so that we need not wonder at our inability to perceive them, either with our whole organism or with any specialised organ. Whatever has no influence upon our welfare as a species can never have any effect upon the modification of our senses.
We can dimly understand, then, why these three kinds of æther-waves, differing from one another only in their relative size and frequency, should be commonly thought of as such utterly unlike agencies. The slowest waves affect all material substances alike, and are consequently cognised by our whole bodies as heat. The middle waves are cast off
in varying proportions by almost every substance upon which they fall, but possess little power of modifying their arrangement, and are consequently cognised by a very special organ — the eye; while the quickest waves are almost inert, so far as our present purpose is concerned, and are consequently not cognised by us at all, except mediately and intellectually.
And now that we have seen the objective nature of light in general, let us ask what is the objective nature of colours in particular.
As I said above, each colour bears objectively the same relation to light as light itself, heat, and chemical rays bear to the whole set of ætherial undulations.
If, once more, we have recourse to the prism and the darkened room, we can throw a bundle of æther-waves as before upon a white screen. Neglecting now the two extremes, the heat-rays and the chemical rays, which are of course invisible, we need only concern ourselves with the middle or light-rays, which form a bright band of colours, ranging from red to violet. The lowest part of this band or spectrum, next to the place where the thermopile showed us the existence of the heat-rays, is occupied by red. After it, in ascending order, come orange, yellow, green, and blue; while the highest place, next to the point where the sensitised paper showed us the existence of the chemical rays, is filled by a belt of violet. Each of these colours answers to a set of æther-waves, whose frequency is intermediate between that of heat-rays and chemical rays in the order just given. Slowest of all visible rays are the red, next come the green and blue, while the violet are the quickest waves capable of producing any direct effect upon the eye.
In the case of such a solar spectrum, we have sifted out the various orders of æther-waves by means of their varying refrangibility, that is to say, the extent to which each is capable of being bent aside from its direct course by means of the prism. But there are other ways in which the same effect may be produced. For example, we may intercept the whole bundle of compound undulations with a piece of specially prepared glass, (red glass, as we call it), which sifts out all the quicker waves, leaving only the red, just as the rock-salt sifted out all the heat-waves. Similarly, we may take a piece of green, blue, or violet glass, which will cut off all but the proper kind of waves which it is intended to let through. Neither of these ways, however, is a common one in external nature. The rainbow shows us the solar spectrum, and the green light which has passed through a stratum of water gives us an instance of selective absorption; but the way in which ordinary colour is produced is a slightly different one.
We saw above that every æther-wave has its origin in an incandescent body, celestial or mundane. But most of the objects which we see every day are not themselves incandescent; the light by which we perceive them is reflected from the sun. Now when the light-waves from the sun strike upon any terrestrial object, they may be reflected in a great many different manners. If the surface upon which they fall is perfectly smooth and quite opaque (or incapable of transmitting the undulations through its substance), the waves will be returned in their entirety, as when we see an image of the sun in a mirror. Here the waves are sent back as they came, exactly in the same way as when a ball rebounds from the wall. If, however, the surface is not quite smooth, but yet has no special selective power for any one set of waves rather than another, the light is then returned, not directly as it came, but dispersedly in every direction. Such an object is said to be white, and its mode of treating the light may be compared to the case of a stone thrown against a wall, and shivered in every direction into a thousand pieces. Again, if the surface has such a molecular disposition that it absorbs or neutralises one or more sets of waves, and only returns one or more other sets, then it is said to be coloured. If it absorbs all the green, blue, and violet rays, returning only the red, then it is said to be a red object, because the red rays alone strike our eyes when we look at it. Similarly, if it absorbs all the red, orange, and violet rays, returning only the green, it is said to be a green object. And so on throughout. Lastly, if it absorbs all the æther-waves, degrading their light into the form of heat, and returning none, it is said to be black.
Almost every object upon which the sunlight falls possesses a power of selecting and returning various æther-waves in varying proportions. Were it not so, the sense of sight could never have been developed. If all objects alike absorbed all the rays which fell upon them, then the whole earth would be one unbroken sheet of black, and the only visible things would be the sun and the fixed stars. If all objects alike reflected all the rays which fell upon them, then the whole earth would be one mass of dazzling white, without distinction of shape or colour. But as each object reflects and disperses the light in different ways from every point of its surface, the discrimination of form, of light and shade, and of colour becomes possible. The existence of the two first-named faculties we must take for granted in this work, though we shall have somewhat further to say about them in the succeeding chapter. But the discrimination of colour, the proper subject of our treatise, demands a little more detailed treatment even at this preliminary stage.
By colour-perception, then, we shall understand in the present work the power of discriminating between light-waves having different rates of frequency. If any creature shows by its actions that it is endowed with such a power, we shall say that it possesses a colour-sense. Anything more than this it is impossible to prove. Whether the sensation or mental idea blue, as perceived or thought by a butterfly or a humming-bird, is the same in consciousness with the sensation or mental idea blue as perceived or thought by you and me, we can never know. For, observe, we can never even know, gifted with language as you and I are, whether my perception of blue is the same as yours; far less then can we know this same thing in the case of animals whose minds are so widely diverse as man’s and the butterfly’s, and between whom intercommunication is impossible. But we can know by means of language that certain objective differences which differentially affect me also differentially affect you. And so too we can know, by the testimony of voluntary or automatic action, that these same objective differences which differentially affect us two, in like manner differentially affect birds, fishes, and insects. Such a power of being differentially affected in the particular case of medium æther-waves having quicker or slower rates of recurrence, we call the colour-sense.
Moreover, just as, in spite of this logical and metaphysical difficulty, no two human beings ever seriously and really doubted the practical and essential identity of their respective sensations, so too in the case before us, I think we shall find such a general agreement in the likes and dislikes of taste, smell, sound, and colour, running through two large groups of animals, whose general habits of life coincide in the main, that we shall not hesitate practically to assert the correspondence of our idea of colour with that of beasts, birds, fishes, and insects. That we can prove this correspondence no one could for a moment maintain; but that we should believe it without strict proof, is not, it seems to me, a very dangerous precedent. Rather should we hesitate to introduce into our conception of the uniform order of nature any supposed difference in kind without full and weighty reason.
A few words more, before we close this unavoidably tedious preliminary statement, as to the nature of the colours objectively existent in nature. As a rule, the bundles of æther-waves which fall upon terrestrial objects are not directly reflected (in other words, the world is not made up of innumerable mirrors); nor are they dispersed in their integrity (in other words, the world is not a sheet of snowy white); nor are they all wholly absorbed (in other words, the world is not a pall of sombre black). A few objects have such surfaces as to reflect totally, like looking-glasses, mercury, or calm water; a few others have such a molecular constitution as to disperse the total beam, like snow-white paper and bleached linen; a few more have such a different molecular arrangement as to absorb entirely, like soot, lamp-black, and broadcloth. But most bodies have their molecules so set as to absorb certain amounts or orders of æther-waves and to return certain other amounts or orders. It
is these last of which we generally speak as coloured objects.
Practically speaking, black, white, and grey only differ in the amount of waves which they reflect and absorb, not in their kinds. A black object absorbs nearly all; a white object disperses nearly all; a grey object absorbs some and disperses some, but in nearly equal proportions of the various kinds. These varieties, then, yield us no sensations of colour proper, but rather of light and shade.
But many objects — the vast majority of objects, in fact — do not reflect the various constituents of the total wave-bundle in their entirety or in equal proportions. They have such a molecular constitution that they select from the waves which fall upon them certain special waves, whose frequency is the same as their own natural rate of oscillation, or else a multiple of the same. All others they reject and reflect back upon the æther without. It is these reflected waves which fall upon our eyes and yield us the sensation of colour.
Very few natural objects, again, outside the organic class, yield us pure colours. Most of them are of dull mixed hues, like the various earths, sands, rocks, and clays. A very small and highly prized class of inorganic bodies do, indeed, reflect light of a single sort only, as in the ruby, the topaz, the amethyst, and other precious stones. But, as a rule, inorganic bodies, as found in nature, are dull browns, dingy greys, or muddy whites. When we turn to the organic world, however, we find pure colours — that is to say, æther-waves of single or slightly compounded orders — very prevalent. In the green leaves of trees, the brilliant tints of flowers, the lovely hues of fruits, the wings of butterflies, the feathers of birds, we find colour constantly appearing in very pure forms. We shall see reason in the sequel to conclude that these pure waves, rather than the mixed and confused systems of inorganic nature, have given rise to the perception of colour in animal organisms.
