This method of securing a comparison light is very much better for sun work than any other, as any variation in the light whose spectrum is to be measured affects the comparison light in the same degree. Thus, suppose I interpose an artificial cloud before the slit of the spectroscope, having adjusted the two shadows, it will be seen that the passage of steam in front of the slit does not alter the relative intensities; but this result must be received with caution. [The lecturer then proceeded to point out the contrast colors that the shadow of the rod illuminated by white light assumed.]

I must now make a digression. It must not be assumed that every one has the same sense of color, otherwise there would be no color blindness. Part of the researches of General Festing and myself have been on the subject of color blindness, and these I must briefly refer to. We test all who come by making them match the luminosity of colors with white light, as I have now shown you. And as a color blind person has only two fundamental color perceptions instead of three, his matching of luminosities is even more accurate than is that made by those whose eyes are normal or nearly normal. It is curious to note how many people are more or less deficient in color perception. Some have remarked that it is impossible that they were color blind and would not believe it, and sometimes we have been staggered at first with the remarkable manner in which they recognized color to which they ultimately proved deficient in perception. For instance, one gentleman when I asked him the name of a red color patch, said it was sunset color.

He then named green and blue correctly, but when I reverted to the red patch he said green.

On testing further, he proved totally deficient in the color perception of red, and with a brilliant red patch he matched almost a black shadow. The diagram shows you the relative perceptions in the spectrum of this gentleman and myself. There are others who only see three-quarters, others half, and others a quarter the amount of red that we see, while some see none. Others see less green and others less violet, but I have met with no one that can see more than myself or General Festing, whose color perceptions are almost identical. Hence we have called our curve of illumination the "normal curve."

We have tested several eminent artists in this manner, and about one half of the number have been proved to see only three quarters of the amount of red which we see. It might be thought that this would vitiate their powers of matching color, but it is not so. They paint what they see; and although they see less red in a subject, they see the same deficiency in their pigments; hence they are correct. If totally deficient, the case of course would be different.

Let us carry our experiments a step further, and see what effect what is known as a turbid medium has upon the illuminating value of different parts of the spectrum. I have here water which has been rendered turbid in a very simple manner. In it has been very cautiously dropped an alcoholic solution of mastic. Now mastic is practically insoluble in water, and directly the alcoholic solution comes in contact with the water it separates out in very fine particles, which, from their very fineness, remain suspended in the water. I propose now to make an experiment with this turbid water.

I place a glass cell containing water in front of the slit, and on 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, while 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 Festing 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 6,000 lies in the red, and the wave length 4,000 in the violet. Now 6,000 is to 4,000 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 (¾)5, 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 colored 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, while tobacco smoke when condensed is black, and very minute particles of water are colorless; it matters not what the color 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 homogeneous fluid, though we know that mastic will not dissolve in water.