870 Professor J, C Maxwell [May 17, 



certain conditions, under which, as we have learned from experience, 

 crystals are formed; but what is our share in the act itself? In like 

 manner we may take a seed or an egg^ and place them under cir- 

 cumstances in which they will develope. In either case we are 

 acquainted with the necessary conditions, and we fulfil them. We can 

 do no more. 



The speaker concluded with some remarks on the fact that the 

 tendency of advancing knowledge is to efface the lines of demarcation 

 which have been hitherto drawn between the natural kingdoms. 



[W. S. S.] 



WEEKLY EVENING MEETING, 



Friday, May 17, 1861. 



The Duke of Northumberland, K.G. F.R.S. President, 

 in the Chair. 



Professor J. CiiERK Maxwell, 

 On the Theory of Three Primary Colours. 



The speaker commenced by showing that our power of vision depends 

 entirely on our being able to distinguish the intensity and quality of 

 colours. The forms of visible objects are indicated to us only by 

 differences in colour or brightness between them and surrounding ob- 

 jects. To classify and arrange these colours, to ascertain the physical 

 conditions on which the differences of coloured rays depend, and to 

 trace, as far as we are able, the physiological process by which these 

 different rays excite in us various sensations of colour, we must avail 

 ourselves of the united experience of painters, opticians, and physiolo- 

 gists. The speaker then proceeded to state the results obtained by 

 these three classes of inquirers, to explain their apparent inconsistency 

 by means of Young's Theory of Primary Colours, and to describe the 

 tests to which he had subjected that theory. 



Painters have studied the relations of colours, in order to imitate 

 them by means of pigments. As there are only a limited number of 

 coloured substances adapted for painting, while the number of tints in 

 nature is infinite, painters are obliged to produce the tints they require 

 by mixing their pigments in proper proportions. This leads them to 

 regard these tints as actually compounded of other colours, correspond- 

 ing to the pure pigments in the mixture. It is found, that by using 

 three pigments only, we can produce all colours lying within certain 



