Sir William de Wiveleslie Abney

“ We are apt to forget, when looking at the spectrum, that what the eye sees is not all that is to be found in the prismatic analysis of light. The spectrum, it must be recollected, is not limited to those rays which the eye perceives. There are rays both beyond the extreme violet and below the extreme red, which exist and which exercise a marked effect on the world's economy. ”

“ The small particles of water and dust—and when we say small we mean small when measured on the same scale as we measure the lengths of waves of light—differentiate between waves of different lengths, and scatter the blue rays more than the green, and the green than the red; consequently what the sun lacks in blue and green is to be found in the light of the sky. ”

“ Now a look at the table will show that the sun is very close to the horizon when it becomes ruddy under normal conditions; but that when the light traverses a thickness of eight atmospheres, the blue and violet, and most of the green, are absent, leaving a light of yellowish colour. To traverse eight atmospheres the light has only to come from a point some eight degrees above the horizon. When the sun is near the horizon, it sends its rays not only to us and over us, but in every direction ”

“ Now let us see how we get the brilliant orange of the sky itself. When the evening is perfectly clear and free from mist and cloud, the orange in the sky is very feeble, showing that the intensity depends upon their presence. ”

“ Why is it that we lose the colour of the flowers and recognize the tiny light from this small worm? The reason for the one is that in order for objects which are not self-luminous to be seen at all, light must fall on them and illuminate them, and the light which they reflect may be coloured if they possess the qualities to reflect coloured light. The glow-worm's light is seen, not because it does not emit light in the day-time, but because the eye, being limited in sensitiveness, is unable to distinguish it when it is flooded with the light of day. ”

“ Sources of Light—Reflected Light—Reflection from Roughened Surfaces—Colour Constants.
There is nothing, perhaps, in our everyday life which appeals more to the mind than colour, yet so accustomed are the generality of mankind to its influence that but few stop to inquire the "why and wherefore" of its existence, or its cause. To those few, however, there is a source of endless and boundless enjoyment in its study; for in the realms of physical and physiological science there is perhaps no other subject in which experiments give results so fascinating and often so beautiful. ”

“ Compound colours, which it may be said are invariably to be found in nature, being mixtures of simple colours, can be just as readily referred to the spectrum. By the luminosity of a colour we mean its brightness, the standard of reference being the brightness of a white surface when illuminated by the same white light. By the purity of a colour we mean its freedom from admixture with white light. ”

“ In the high Alps it is no uncommon occurrence for the snow-clad mountains to be tipped with the same crimson we have described as colouring the clouds, and this is usually just after sunset, when the sun has sunk so low beneath the horizon that the light has to traverse a greater thickness of dense air, and consequently to pass through a larger number of small particles than it has when just above the horizon. In this case the red of the sunlight mixes with blue light of the sky, and gives us the crimson tints. ”

“ The effect that small water particles have upon light passing through them can be very well seen in the streets of London at night, when the atmosphere is at all foggy. Gaslights at the far end of a street appear to become ruby red and dim, and half-way down only orange, but brighter, whilst close to they are of the ordinary yellow colour, and of normal brightness. ”

“ All these artifices—or rather we should say intuitive applications of science—are most necessary when the small range of luminosity of colours with which he has to deal is taken into account. For instance, in a picture of a sun-lighted snow mountain and deep pine forests, the utmost luminosity he can give to the former is that of white paper when seen in the shade, which, in comparison with what he sees, is really a mixture of 90% of black with the light from the snow, so that his range of luminosity is only nine-tenths of that which occurs in nature. ”

“ In the same way if the rate of vibration of a wave of light is in accord with that of a molecule or atom, the amplitude or swing of the atom or molecule is increased, and the energy of the wave and therefore its amplitude is totally or partially destroyed; and as the amplitude is a function of the intensity of the light, the ray fails to be seen at all, or else is diminished in brightness. ”

“ How is it that the combination of three colours suffices to form white, or to match any colours we wish, be they spectrum colours to which a little white is added, or the colours of pigments? The most plausible theory that can be advanced is that it is only necessary for the eye to be furnished with a three-colour-perceiving apparatus to give the impression of every colour, and yet this would be somewhat difficult to believe had we not had the experiments narrated in that chapter before us. ”

“ We will now turn to a phase of colour measurement which will materially help us to see why, in some cases, the addition of white light to the simple spectrum colours, between the red and green, does not appear necessary in order to make a match with a mixture of red and green.
We will ask ourselves two questions: one is, whether any colour, and if so how much, can be added to white without appearing to the eye? ”

“ We have a green dye which to the eye is very similar in colour to chlorophyll, but putting a solution of it in front of the spectrum, we see that it cuts off different rays to the latter. It would be quite possible to mistake one green for the other, but directly we analyze the white light which has filtered through each by means of the spectrum, we at once see that they differ. Hence the spectrum enables the eye to discriminate by analysis what it would otherwise be unable to do. ”

“ The colour of an object in nature, without exception we might almost say, is due, not to one simple spectrum colour, or even to a mixture of two or three of them, but to the whole of white light, from which bands of colour are more or less abstracted, the absorption taking place over a considerable portion or portions of the spectrum. ”

“ That a candle-flame or the sun is self-luminous is an axiom, and need not be philosophised upon; but what must be impressed on the reader is, that though an object which requires to be illuminated to be seen, is not self-luminous, yet when illuminated it does in fact become a source of illumination to the eye, although the light is only light reflected from its surface. It is a point worth remembering that the rougher the surface of an object, the brighter to the eye it will be. ”

“ A spectrum may be produced from any source of light, such as sunlight, limelight, the electric light, gaslight, or incandescence electric light, as also from incandescent vapours, or gases; but it is only a solid which is, or is rendered incandescent, that will give us a continuous spectrum, as it is called, that is, a spectrum which is unbroken by gaps of non-luminosity, or sudden change of brightness, throughout its length. ”

“ There is a phenomenon in colour which must be alluded to, and which possesses more than a passing interest to the art world, and that is colour contrast. Perhaps one of the best methods of showing this is by our colour patch apparatus. If we throw the reflected beam and the colour patch on a square as before, and place a rather thinner rod in front, so that the two shadows lie on a background of the combined white light and spectral colours, on passing a slit through the spectrum, the shadow which is illuminated by white light will appear anything but white. ”

“ Suppose in the eye we have three such sensitive pendulums which are capable of responding to the beats of waves of light, it requires no great imagination to see that one such pendulum will respond not only to that wave of light which is isochronous with it, but also with waves shorter and longer than that particular wave. ”

“ The crimson on the clouds, illuminated by the setting sun, would appear to him as only slightly different to the white clouds which he would see at midday; in fact he would be always seeing what to us would be a sunset. For this gentleman [Pg 146] to mix spectrum colours to match others would evidently be no guide to normal-eyed persons.
We believe that amongst us in our daily life we have many persons who are blind to some colour, but who are not aware of it, or if they are aware of it, hide their defect as far as possible. That some are ignorant of it to a late period of their life we know. ”

“ Scattered Light—Sunset Colours—Law of the Scattering by Fine Particles—Sunset Clouds—Luminosities of Sunlight at different Altitudes of the Sun.
It is probable that we should be able to ascertain approximately the true colour of sunlight (if we may talk of the colour of white light) if we could collect all the light from a cloudless sky, and condense it on a patch of sunlight thrown on a screen. For skylight is, after all, only a portion of the light of the sun, scattered from small particles in the atmosphere, part of the light being scattered into space, and part to our earth. ”

“ For some purposes, as for instance in becoming acquainted with the general character of the spectrum, a feebler light, such as gaslight, or light from electrical glow lamps, may be employed, since the spectrum may be viewed directly by the eye without the intervention of a screen. They have two drawbacks for our object: one being the want of general intensity, and the other the feeble luminosity of blue and violet rays in their spectrum (see page 110) . The limelight we can also dismiss for want of steadiness. ”

“ On the other hand, if the spectrum be diminished in brightness the tendency is for the colours to change in the opposite direction. Thus the yellow almost disappears and becomes of a green hue, whilst the orange becomes redder, and the spectrum itself becomes shorter to the eye than before.
Let us strictly guard ourselves, however, from the criticism that all eyes see not alike. Suffice it to say that the above table is correct for the ordinary or normal eye, and does not necessarily apply to those who have defective vision as regards colour sensation. ”

“ There are two variable factors which have to be taken into account in colour experiments: the first is the quality of light which illuminates the object, and the second [Pg 15] is the sensitiveness of the eye which perceives it, as light is only a sensation which is recognized by the brain through the medium of the eye. We shall, as we go on, see that different qualities of light may cause objects to appear of different hues, and further that eyes may vary in perceptive power, to an extent of which the large majority of people are not aware. ”

“ A very crucial experiment is to see if we can make white light by the admixture of three colours, for if this can be done it almost follows that any colour can be formed. We must use the colour patch apparatus, and begin with placing one slit in the violet near the line G, another between E and F, and a third between B and C of the solar spectrum, and fill up the gaps between them with cards as shown in the figure. For our present purpose it is better to make the colour patch and the white patch touch each other, not using the rod, as by this means we avoid fringes of colour. ”

“ When no fog is present the gas-lights in the distance and close to are of the same colour and brightness, showing that their change in appearance is simply due to the misty atmosphere intervening between them and the observer. We can imitate the light from the sun, after its passage through various thicknesses of atmosphere, in a very perfect manner in the lecture-room, using the electric light as a source. A condensing lens is put in front of the lamp, and in front of that a circular aperture in a plate. ”

“ It will probably happen that a good deal more of one of the pairs of the colours is required in the disc than of the other, and supposing that the two are each used of the full brightness which the pigments are capable of giving, it follows that in a diagram where equal areas are filled with the pigments as complementary, some means must be adopted to give the true depth of tone to each. The mixture of white will heighten the luminosity of either, or the admixture of black will lower it, but often alters the hue. ”

“ As we have arrived at the fact that in colour mixtures of violet and green, white light is to be found in the colour produced, it follows that either the violet or the green, or both, must themselves contain some small proportion of white. ”

“ We must repeat once more that the blue of the sky is principally if not entirely due to the presence of these particles, the rays scattered by them, which are principally the blue rays, being reflected back from them, giving the sensation of blue which we know as sky-blue. The greater the number of these fine particles that are encountered by sunlight, the greater the scattering will be, and the bluer the sky. ”