due to the undulations of a medium called the luminiferous ether which is assumed to pervade all space. There is no direct evidence of this ether, but the theory explains in such a complete manner all the various phenomena of light, that it is hardly possible to doubt its correct

ness.

There is considerable difference between the undulations of air in the production of sound, and those of the luminiferous ether giving rise to the sensation of light. In the former case the waves are waves of condensation and rarefaction; that is to say, the direction of the motion has the same direction as the sound. In the case of light, the wave motion is perpendicular to the path of the ray. The vibrations of a ray of light are in all azimuths, but the movement of the wave is in each case perpendicular to the path of the ray. Wave-motion may be admirably illustrated by throwing a stone into a pond. The stone will, for a short period of time, cause a depression in the surface of the water. On account of this depression the adjacent molecules of water will be raised above the horizontal level of the pond, and a wave will be started. The molecules forming this wave will then sink to the level of the pond and then below it, the adjacent molecules being influenced as in the first instance. In this way the wave will be propagated until either the bank is reached or the waves become too feeble to influence the molecules of water adjacent to them. It will be noticed that there is no real transmission of matter; thus, if there be a stick or leaf in the path of the wave, it will be raised with the wave, but left in the same spot after the wave has passed.

The following table* gives the length in inches of the undulations corresponding to the light at the principal dark lines of the spectrum :—

* Ganot's "Physics," p. 586.

The following is a table of wave-lengths in tenth

The velocity of light is 300 million metres per second, or 300 x 1016 tenth-metres per second. The number of waves per second for any colour is therefore 300 × 1016 divided by its wave-length as above expressed. Proceeding in this manner we find approximately :

For A, 395 millions of millions per second.

The respective vibrations for the rays at the ends of the spectrum are as follows. For the extreme red, 395 billions of vibrations per second. For the extreme violet, 763 billions of vibrations per second.

The rays of light which give rise to the sensation of red are also less refrangible than the remainder, the refrangibility increasing up to the violet.

From this it will be seen that a spectrum consists of an enormous number of rays of light arranged side by side, each differing from the others in wave-length and refrangibility.

Besides the rays of light which compose the visible

spectrum, there are two other sets of rays, which under ordinary circumstances are not visible to the naked eye. These are the heat-rays and the actinic rays. The former are of lower refrangibility and have a longer wave-length than the red rays; the latter possess greater refrangibility and a shorter wave-length than the violet rays.

Under certain circumstances these rays may be transmuted that is, have their wave-length altered, and so be made visible. Professor G. G. Stokes has succeeded in converting the ultra-violet into rays of lower refrangibility, whilst Professor Tyndall has been able to convert the infrared into rays of a higher degree of refrangibility. If the spectrum be looked at through a piece of cobalt glass, a bright crimson band will be seen below the red, whilst the presence of rays above the violet is demonstrated by fluorescence and other phenomena.

The most important point concerning the physical basis of colour is, that the rays of light giving rise to the sensation are arranged in a series in the spectrum, and that physically we are not cognisant of the limits of this series. We know that there are heat-rays and actinic rays because we have direct evidence of their existence, but we do not know where the light series commences or terminates. There may be rays considerably below the red which are performing useful work, and which cannot be brought under the direct evidence of the senses. In order that the reader may be able to follow the theory on which this book is based, it is essential that he should regard the spectrum as a portion of a physical series, the latter having no commencement, termination, or definite unit. The physical-light series, therefore, consists of rays having different wave-lengths. A unit in the physical series is represented by a single ray having a definite wave-length. The physical basis of colour is the light

which is reflected from or transmitted by a substance. Without these rays a substance will not only not be coloured, but appear black. For instance, if we look at a piece of ultramarine by the light of burning sodium it appears black. Without light we cannot have colour, and as the light of the sun is white, it must follow that something must be added to, or subtracted from, this light before colour can be produced. In the following chapters it will be shown why a certain combination of rays gives rise to the sensation of a certain colour. In this chapter it only remains for us to consider what changes in the series are capable of giving rise to any sensation of colour. The following are the ways in which white light is altered so as to give rise to the sensation of colour.

1. Colours produced by Dispersion. When colours are produced in this way, the white light is not altered in character. The constituent rays, on account of their unequal refrangibility, are spread out in the form of a series. This is called the dispersion of white light. The colours of the rainbow are good examples of colours produced by dispersion.

2. Colours produced by Absorption.-In the case of opaque bodies a certain number of rays of light are absorbed, and the remainder reflected, giving rise to a sensation of colour. In the case of transparent bodies, a certain number of rays are absorbed, and the remainder transmitted, giving rise to a sensation of colour.

This is the commonest way in which colours are produced; the colours of flowers and pigments of all kinds being due to this cause. When pigments are mixed the resulting colour is that which is reflected by both pigments. It is in this way that yellow and blue pigments, when mixed, make a green. The pigments used are not pure-that is to say, the yellow pigment reflects green as

If a piece of yellow a white cloud viewed

well as yellow light, and the blue pigment reflects green as well as blue light. When the two are mixed, the whole of the light with the exception of the green is absorbed, and so the resulting colour is green. A very simple experiment will demonstrate this fact. and a piece of blue glass be taken and through the two it will appear green. The piece of yellow glass may then be subjected to spectroscopic analysis: it will be found to transmit in addition to the yellow rays the orange, green, and some of the red rays. The blue glass will be found to transmit, in addition to the blue rays, the violet, green, and a band of red at the extreme left of the spectrum. If the light transmitted by both glasses be now examined, it will be found to consist almost entirely of the green rays, hence the green colour of the transmitted light.

3. Colours produced by Interference. The production of colour by interference is admirably explained by the wave theory of light. The following explanation will show how these colours are produced. If we throw a stone into a pond we shall start a series of waves. If, before these waves have subsided, we throw another stone into the pond, in a different place, we shall set up another series of waves. The following phenomena will then be noticed. Where the waves meet so that the crest of one wave corresponds to the crest of another wave, the two will combine, giving rise to a wave twice the size of either. If, however, the waves meet so that the crest of one wave will correspond to the trough of another, the water will remain calm and undisturbed. The light-waves may interfere in a similar manner. When the crests of the light waves correspond, both are reinforced, and the light becomes brighter. When the crest of one light wave corresponds to the trough of another of similar wave