CHAP. II. FUNCTION OF NUTRITION. 199 



This colour therefore is one of forty-eight pure co- 

 lours which would compose a fifth circle constructed 

 on the plan alluded to. We may remark that any 

 two colours arranged in opposite compartments added 

 together make white or grey, and are hence styled 

 complementary colours. Thus (2B + Y) is exactly 

 opposite to (2 R + Y), and these added together 

 make up (2 B + 2 R + 2 Y) or 2 N ; and so of any 

 others. 



(185.) Impure Colours. From what we have said 

 it appears, that every tertiary or other compound among 

 material colours, that is to say every dull or " impure" 

 colour, must be some pure colour mixed with a greater 

 or less proportion of grey. Thus, a colour com- 

 posed of (9 B + 7 Y + 4 R) is the same as (4 B + 4 

 1T-M R) 4 (5 B + 3 Y), which is the same as (4 N) 

 -f- (5 B -f- 3 Y), or a combination of grey (4 N) with 

 the pure colour represented by (5 B + 3 Y) which is 

 one of the bluish greens. Many ternary compounds have 

 obtained specific names; thus the different " browns" 

 result from various proportions of grey mixed with some 

 pure colour of which red is a constituent part ; and the 

 " Olives" are some of the greens similarly rendered 

 impure. 



In order to conceive how every possible impure colour 

 may be formed by combining the pure colours with 

 grey, we may take the deepest shades of all the former 

 and having placed them in the compartments of a circle 

 divided as before, combine them with all the shades 

 of grey beginning with the palest in the centre and 

 proceeding to the darkest in the circumference ; and 

 then in another circle concentric with the former, com- 

 bine every shade of all the brilliant colours with the 

 deepest shade of grey. This double arrangement gives 

 us every possible mixture between the basial colours and 

 grey; that is to say every possible ternary compound or 

 impure colour. Thus in the annexed figure (155.), if the 

 deepest shade of blue extends from (a) to (6), and the 



