THEORY OF MIXED COLOURS. 79 



mixed with a few drops of water and pieces of cardboard 

 covered with the paint so obtained, and then dried. But if 

 we take an intimate mixture of two powders, and make it 

 into a paint with oil or water, the gravity of the two powders 

 being different, and the fluid medium imparting a certain 

 degree of mobility to the particles, there will be a tendency 

 for the lightest powder to come to the surface. I have 

 sometimes noticed, with regard to the tablets prepared as 

 above, that portions which had been rubbed seemed 

 perceptibly lighter than the undisturbed portion ; possibly 

 this may be due to a slight excess of carbon on the upper 

 surface. 



I also compared tint A with another consisting of 0*4003 

 gram carbon to 10 grams BaS0 4 ; thus the quantity of 

 carbon is a considerable multiple of that contained in tint 

 A. The value of R, got from the comparison differed con- 

 siderably from a value of E, which I got by comparing A 

 with B. At first I thought this was due to a failure in 

 the theory ; after some time it occurred to me that the 

 conditions of my experiments were not the same as the 

 conditions of the theory. In the theory I had supposed 

 that the white was mixed with a perfect black ; in the ex- 

 periments the white had been mixed with grey. 



Those surfaces which are popularly known as black are 

 in reality not black but grey. If we take such a surface, 

 whether of black velvet or lampblack, and hold an opaque 

 object before it so as to intercept a portion of the incident 

 light, a shadow will be found on the surface ; but it is 

 evident that a perfect black is incapable of receiving a 

 shadow. Also, when I looked at a surface of lampblack 

 through the colorimeter (consisting of a glass cylinder 

 covered with black cloth, except a small aperture at the 

 bottom) the lampblack surface appeared grey. The 

 formula for the intensity of the residual whiteness, if we 



