March 24, 1887] 



NA TURE 



499 



rays from P, fall on G, a silver-on-glass mirror. They are col- 

 lected by L5, and form a white image of the prism als^ at F. 

 The method we can adopt of altering the intensity of the 

 comparison light is by mean-; of rotating sectors, which can be 

 opened or closed at will, and the two shadows thus ma'le equally 

 luminous. [Shown.] Kut althou;»h this is an excellent plan 

 for some purposes, we have found it better to adopt a different 

 method. You will recollect that the brightest part of the 

 spectrum is in the yellow, and that it falls ofT in brightness on 

 each side, so, instead of opening and closing the sectors, they are 

 set at fixed inten-als, and the slit is moved in front of the spec- 

 trum, just making the shadow cist by the reflected beam too 

 dark or too light, and oscillating between the two till equality is 

 discovered. The scale number i-; then noted, and the curve 

 constructed as before. It must be remembered that, on each 

 side of the yellow, equality can be established. 



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 afiects the comparison lij^ht 

 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 hcturer then proceeded to 

 point out the contrast colours that the shadow of the rod 

 illuminated by white light assumed.] 



I must now make a digression. It must not be assumed that 

 everyone has the same sense of colour, otherwise there would 

 be no colour-blindness. Part of the researches of General Fest- 

 ing and myself have been on the subject of colour-blindness, 

 and these I must briefly refer to. We test all who come by 

 making them match the luminosity of colours with white light, as 

 I have now shown you ; and as a colour-blind person has only 

 two fundamental colour-perceptions instead of three, his match- 

 ing 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 cjlour-peroeption. Some have remarked that it is im- 

 possible that they were colour-blind, and would not believe 

 it, and sometimes we have been staggered at first with the 

 remarkable manner in which they recognised colour to which 

 they ultimately proved deficient in perception. For instance, 

 one gentleman when I asked him the name of a red colour 

 patch, said it was sunset colour ; 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 

 colour-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, whilst 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 

 Testing, whose colour-perceptions are almost identical. Hence 

 \\e 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 m itching colour, 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, aixi see what eflect 

 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. 

 NoA- mastic is practically insoluble in w.iter, 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 1 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 meaJure 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 exa- 

 mining the blue patch of light is placed in front of the slit, 

 much more of this light is'allswed to pass than of the blue. If 

 we measure the light we shall find that the same law hold-; 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 theo- 

 retical 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 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 l6, or 

 as about 5 to i. If, then, the four inches of our turbid medium 

 allowed three-quarters of this particular red ray to be transmitted, 

 they would only alloy (|)°, 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 panicles 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 minute 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 i/iooo of a cubic 

 inch of mastic varnish, and the water in it measures about 

 103 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. 

 Now a wave-length in the red is .about 1/40,000 of an inch, and 

 a little calculation will show that these particles are well within 

 the necessary limits. Prof. Tyndall has delighted audien:es 

 here with an exposition of the effect of the scattering of light 

 by small particles in the formation of artifical skies, and it would 

 be superfluous for me to enter more into that. Suffice it to say 

 that when particles are small enough to form the artificial blue 

 sky they are fully small enough to obey the above law, and that 

 even larger particles will suffice. We may sum up by saying 

 that very fine particles scatter more blue light than red light, 

 and that consequently more red light than blue light passes 

 through a turbid medium, and that the rays obey the law pre- 

 scribed by theory. I will exemplify this once more by u<ing the 

 whole spectrum and placing this cell, which contains hypo- 

 sulphite of soda in solution in water, in front of the slit. By 

 dropping in hydrochloric acid, the sulphur separates out in 

 minute particles ; and you will see that, as the particles increase 

 in number, the violet, blue, green, and yellow disappear one by 

 one and only red is left, and finally the red disappears itself. 



Now let me revert to the question why the sun is red at sun- 

 set. Those who are lovers of landscape will have often seen on 

 some bright summer's day that the most beautiful effects are 

 those in which the distance is almost of a match to the sky. 

 Distant hills, which when viewed clo3e to are green or brown, 

 when seen som^ five or ten miles away appear of a delicate and 

 delicious, almost of a cobalt, blue colour. Now, what is the 

 cause of this change in colour ? It is simply that we have a sky 

 formed between us and the distant ranges, the mere outline of 

 which looms through it. The shadows are softened so as almost 

 to leave no trace, and we have what artists call an atmospheric 

 eflfect. If we go into another climate, such as Egypt or amongst 

 the high Alps, we usually lose this effect. Distant mountains 

 stand out crisp with black shadows, and the want of atmosphere 



