70 DR. JAMES BOTTOMLEY ON A 



VIII. Colorimetry. — Part VI. 

 By James Bottomley, B.A., D.Sc. 



Bead October 5th, 1880. 



On a Theory of Mixed Colours. 



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 degree 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 pro- 

 portions 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 

 so by additional water ; but the painter in oil or distemper, 



