PROCEEDINGS 
OP THE 
LITERARY AND PHILOSOPHICAL SOCIETY. 
Ordinary Meeting, October 5th, 1880. 
R Angus Smith, Ph.D., F.RS., &c., Y.P., in the Chair. 
“ Colorimetry, Part VI. On a Theory of Mixed Colours,” 
by James Bottomley, B. A., D.Sc. . 
In some experiments which I was making on the absorp- 
tion of light, I needed surfaces of different degrees of 
whiteness. To obtain such surfaces I mixed black and white 
powders in various proportions. They were blended by 
shaking in a bottle and grinding in a mortar. In some cases 
the mixture was reduced to a flat surface by pressure, in 
other cases, a little water was added so as to form a paint, 
and with this pieces of cardboard were covered and dried. 
Although it was not necessary for my purpose that I should 
quantitatively assign the degrees of whiteness in each case, 
yet such a question occurred to me in the preparation of 
these powders. In this matter little has, I think, been done. 
Newton, in his Optics, informs us that he mixed powders of 
different colours in such proportions as to form a grey, and 
in later times it has been proposed to estimate the degree 
of blackness of different bodies by finding the quantity of 
some white powder, which, on admixture, will yield some 
standard tint. But no one has I think ever considered the law 
of intensity of colour when we mix a colour with white or 
black. Such a question is interesting to the artist, to the 
colour mixer, and to the chemist. The artist in water- colour, 
if he wishes to reduce the strength of any pigment, can do 
Proceedings—Lit. & Phil, Soc.—Yol, XX.—- No. L— Session 1880-L 
