196 PHYSIOLOGICAL BOTANY. PART II. 



referred to different degrees of mixture between three 

 colours, which are considered as " primary." These 

 we may assume to be red, blue, and yellow. A mix- 

 ture of red and blue makes purple ; of red and yellow 

 makes orange ; of blue and yellow makes green ; and 

 innumerable binary compounds may be formed by unit- 

 ing the primaries two and two in different proportions. 

 Innumerable shades also of each of these may be ob- 

 tained, between the deepest that can be formed and 

 the faintest, by diluting each colour to a greater or 

 less extent. In order that we may consider every 

 colour to be formed on some regular principle, we 

 divide a circle into three equal parts (fig. 154. in. 

 nermost), and place the Blue (B), Red (R), and Yellow 

 (Y), in each of them re- 

 spectively. Around this 

 circle a second is de- 

 scribed, and divided into 

 six equal compartments 

 containing respectively 

 the three primaries, and 

 also those three binaries 

 which are exactly inter, 

 mediate between the"m ; 

 viz. the Orange (R + Y), 

 the Purple (B -f R), and 

 the Green (B + Y) ; as- 

 suming these also of the same shade as before. Another 

 circle containing twelve equal compartments is described 

 round the last, and in these are placed the last six 

 colours, together with six new ones formed by uniting 

 each contiguous pair in the same way as before. An- 

 other circle would contain twenty-four colours and 

 so on ; each fresh addition being always formed from 

 the combination of two contiguous colours in a former 

 circle, and between which it is to be exactly inter- 

 mediate ; and the whole is then reduced to a uniform 

 shade. By proceeding in this way it is evident that 

 we may form every conceivable binary compound, or 



