OTHER CAUSES OF COLOUR.
The colours of the spectrum are to the sense of sight
what the tones of the gamut are to the sense of hearing.
On the one hand, the differences in the lengths
of the sonorous waves constitute the variety of note perceptible
by the ear; on the other, the differences in the
lengths of the luminous waves constitute the variety of
colour perceptible by the eye. By and by, we shall
learn both the length and rapidity of these vibrations,
but it will be as well first to describe the experiments
made in this direction by the immortal Newton
himself.
Every one has, doubtless, at one period of his life, amused himself with blowing soap-bubbles by means of a tobacco-pipe and a little lather—a sufficiently childish amusement, you will possibly say, but one narrowly connected with the most intricate secrets of the science of optics. These little globes, so fragile that they disappear in a breath, hardly seem worthy of the attention of a thinker, and still less the examination of a philosopher; but it is nevertheless true that Newton made experiments on the colours shown on the surface of these apparently insignificant objects which ended in the most brilliant discoveries, just as on seeing an apple fall he began a train of thought which only terminated in the enunciation of the hypothesis of the earth's power of gravity.
All transparent substances, whether liquid, solid, or gaseous, become coloured with the most brilliant hues as soon as they are reduced to plates of extreme thinness. In the soap-bubble it is the oleaginous particles floating on the surface which thus become coloured, but Newton showed that thin plates of air were similarly capable of showing colour, and that the thinner the plates were the more brilliant were the tints. We may see this in the soap-bubble, which becomes more beautiful as it gets larger and thinner. By placing a convex lens of large size on a flat plate of glass, Newton observed that rings of different colours were formed round the spot where the two pieces of glass touched.
Fig. 12.—Newton's Rings.
By measuring the convexity of the lens and the diameter of the various rings, Newton was enabled to tell to a minute fraction the exact thickness of the plate of air corresponding to the different colours. The glasses being placed in position, a ray of a particular colour—red, for instance—was thrown upon the surface. The result was a black spot at the point where the two surfaces touched, and surrounding it at various distances were several rings alternately red and black. Calculating the thickness of the plates of air at the part where the dark rings made their appearance, Newton found that their dimensions were in the proportion of the even numbers two, four, six, eight, &c.; while the red rings showed figures corresponding to the odd numbers. Although trammelled by the corpuscular theory, Newton's deductions from these experiments show that they can only be accounted for by the undulatory hypothesis. Thus the thickness of the plate of air at the first red ring is that of the red wave, the thickness at the second that of two red waves, and so on; so that in order to arrive at the thickness of the red wave we need only measure the distance between the portions of the glasses where the first red ring occurs.
This experiment, was applied to the measurement of all the waves. Whenever they were reflected on the glasses a parallel series of rings was formed, but it was found that the first ring was more or less distant from the central spot, according to the colour used. The red ring was the largest; the orange, yellow, green, blue, indigo, and violet, following in the same sequence as in the spectrum. The word "thickness" seems hardly fit to apply to dimensions arrived at by Newton in his experiments, so infinitely small do they appear to be, yet their correctness has never been impugned, although the experiments have been repeated by the philosophers of all countries. The waves of red light are so small that 40,000 of them go to an inch, and those of violet light situated at the other end of the spectrum are still smaller, measuring only the 60,000th part of an inch.
The waves of the other colours are between these two, while the wave of white light, which is a mixture of them all, is just half-way between the two.
Thus was the physical cause of the various hues of colour discovered by this great man, revealing as it does the singular and mysterious analogy between sound and light. The rays of light, like the waves of sound, produce a different effect, according to their length, by causing quicker or slower pulsations in the nerves of sight, just as musical sounds vibrate upon the drum of the ear with different velocities.
This is not all, for the relationship between sound and light does not cease here: we have as yet only spoken of the size of the undulations, and have only shown how their dimensions are connected with the sensation of colour; but there are other things to be considered, for on investigation we find that not only do the different coloured waves vary in the length of their undulations, but also in the number that take place in a given time.
The perception of sound is produced by the action of the drum of the ear, which vibrates sympathetically with the pulsations of the air that have been originated by the vibrations of the sounding body; and the perception of light is produced in a similar manner by the vibrations originating in a luminous body, and propagating themselves through the luminous ether until they reach the nerves of sight. The number of these pulsations taking place in the eye has been accurately determined in the following manner. Let us suppose that we are looking at a coloured object—let us say, a red railway signal-lamp; from the lamp to our eye there flows a continuous line of luminous undulations; these undulations enter the eye and become depicted on the retina. For every wave that passes through the pupil, there is a separate and corresponding vibration of the optic nerve, and the number of these vibrations that take place in the course of a second can be easily calculated if we know the velocity of light and the breadth of the waves. We have before found that light travels at the rate of 185,000 miles per second; it therefore follows, that a series of undulations 185,000 miles long pass through the pupil every second; consequently the number of vibrations per second is arrived at by calculating how many waves measuring the 40,000th of an inch—that being the length of a wave of red light—are contained in 185,000 miles. The following table, showing the number of waves passing into the eye per second for the different colours, will interest the student:—
| Extreme red | 458,000,000,000,000 | waves per second. |
| Red | 477,000,000,000,000 | waves per second.„ |
| Orange | 506,000,000,000,000 | waves per second.„ |
| Yellow | 535,000,000,000,000 | waves per second.„ |
| Green | 577,000,000,000,000 | waves per second.„ |
| Blue | 622,000,000,000,000 | waves per second.„ |
| Indigo | 658,000,000,000,000 | waves per second.„ |
| Violet | 699,000,000,000,000 | waves per second.„ |
| Extreme violet | 727,000,000,000,000 | waves per second.„ |
Whatever theory we may adopt to explain the phenomena of light, we arrive at conclusions that strike the mind with astonishment and admiration. According to the corpuscular hypothesis, it was supposed that the molecules of light were endowed with the power of attraction and repulsion, that they possessed poles and centres of gravity like the earth, and that they had other physical properties that could only be given to ponderable matter. Starting with these notions, it is difficult to divest oneself of the idea of sensible size, or to induce the mind to conceive particles so extremely small as those of light would necessarily be if the theory of emission were accepted. If a particle of light weighed a grain, it would produce by means of its enormous velocity the effects of a cannon-ball weighing 120 lbs., travelling at the rate of 300 yards per second. How infinitely small would be these particles, seeing that the most delicate optical instruments are submitted to their action for years without being injured!
If we are astonished at the extreme smallness and prodigious rapidity of the luminous molecules whose existence is necessitated by the corpuscular theory, the numerical results of the undulatory hypothesis are not less surprising. The extreme smallness of the distance between the waves, and the inconceivable quickness of their undulations, although both are easily calculated, must raise in the mind of the student feelings of the utmost wonder and admiration.
Colour, then, simply results from the difference in the rate of vibration of the rays, as Professor Tyndall observes in his lectures on the "Analogy between Sight and Sound," the impression of red being produced by waves that undulate a third less rapidly than those which produce the sensation of violet.