64 • Ca2)tam W. de W. Ahney [Feb. 25, 



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 colours 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 colour, otherwise there would be no 

 colour-blindness. Part of the researches of General Testing 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 per- 

 ceptions 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 colour-perception. Some have remarked that it is 

 impossible 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 defi- 

 cient 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 sj)ectrum 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 Festiug, whose colour-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 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, and see what effect 

 what is know^n as a turbid medium has upon the illuminating value 

 of difierent 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 w^ater 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 



