1887.] on Sunlight Colours. 65 



the screen I throw a patch of blue light. I now change it for turbid 

 water in a cell. This thickness much dims the blue ; with a still 

 greater thickness the blue has almost gone. If I measure the 

 intensity of the light at each operation, I shall find that it diminishes 

 according to a certain law, which is of the same nature as the law of 

 absorption. For instance, if one inch diminishes the light one-half, 

 the next will diminish it half of that again, the next half of that 

 again, whilst the fourth inch will cause a final diminution of the 

 total light of one-sixteenth. If the first inch allows only one-quarter 

 of the light, the next will only allow one-sixteenth, and the fourth 

 inch will only permit 1/256 part to pass. Let us, however, take a 

 red patch of light and examine it in the same way. We shall find 

 that, when the greater thickness of the turbid medium we used 

 when examining the blue patch of light is placed in front of the slit, 

 much more of this light is allowed to pass than of the blue. If we 

 measure the light we shall find that the same law holds good as 

 before, but that the proportion which passes is invariably greater 

 with the red than the blue. The question then presents itself: Is 

 there any connection between the amounts of the red and the blue 

 which pass ? Lord Rayleigh, some years ago, made a theoretical 

 investigation of the subject ; but, as far as I am aware no definite 

 experimental proof of the truth of the theory was made till it was 

 tested last year by General Testing, and myself. His law was that 

 for any ray, and through the same thickness, the light transmitted 

 varied inversely as the fourth power of the wave-length. The wave- 

 length 6000 lies in the red, and the wave-length 4000 in the violet. 

 Now 6000 is to 4000 as 3 to 2, and the fourth powers of these wave- 

 lengths are as 81 to 16, or as about 5 to 1. If, then, the four inches 

 of our turbid medium allowed three-quarters of this particular red 

 ray to be transmitted, they would only allow ( j)^, or rather less than 

 one-fourth, of the blue ray to pass. Now this law is not like the law 

 of absorption for ordinary absorbing media, such as coloured glass for 

 instance, because here we have an increased loss of light running 

 from the red to the blue, and it matters not how the medium is made 

 turbid, whether by varnish, suspended sulphur, or what not. It holds 

 in every case, so long as the particles which make the medium turbid 

 are small enough; and please to recollect that it matters not in the 

 least whether the medium which is rendered turbid is solid, liquid, 

 or air. Sulphur is yellow in mass, and mastic varnish is nearly 

 white, whilst tobacco-smoke when condensed is black, and very minuto 

 particles of water are colourless : it matters not what the colour is, 

 the loss of light is always the same. The result is simply due to the 

 scattering of light by fine particles, such particles being small in 

 dimensions compared with a wave of light. Now, in this trough is 

 suspended 1/1000 of a cubic inch of mastic varnish, and the water in 

 it measures about 100 cubic inches, or is 100,000 times more in bulk 

 than the varnish. Under a microscope of ordinary power it is 

 impossible to distinguish any particles of varnish : it looks like a 

 Vol. XII. (No. 81.) f 



