440 LECTURE xxxvri, 



a mixture of red, green, and violet, only, in the proportion of about two 

 parts red, four green, and one violet, with respect to the quantity or intens- 

 ity of the sensations produced. 



If \vc mix together, in proper proportions, any substances exhibiting these 

 colours in their greatest purity, and place the mixture in a light sufficiently 

 strong, we obtain the appearance of perfect whiteness ; but in a fainter light 

 the mixture is grey, or of that hue which arises from a combination of white 

 and black ; black bodies being such as reflect white light but in a very 

 scanty proportion. For the same reason, green and red substances mixed to- 

 gether usually make rather a brown than a yellow colour, and many yel- 

 low colours, when laid on very thickly, or mixed with black, become brown. The 

 sensations of various kinds of light may also be combined in a still more satisfac- 

 tory manner by painting the surface of a circle with different colours,in any way 

 that may be de,sired, and causing it to revolve with such rapidity, that the whole 

 may assume the appearance of a single tint, or of a combination of tints, 

 resulting from the mixture of the colours. (Plate XXIX. Fig. 423 . . 426.) 



From three simple sensations, with their combinations, we obtain seven 

 primitive distinctions of colours ; but the different proportions, in which they 

 may be combined, afford a variety of tints beyond all calculation. The three 

 simple sensations being red, green, and violet, the three binary combina- 

 tions are yellow, consisting of red and green; crimson, of red and violet; 

 and blue, of green and violet; and the seventh in order is white light, composecl 

 by all the three united. But the blue thus produced, by combining the whole of 

 the green and violet rays, is not the blue of the spectrum , for four parts of green 

 and one of violet make a blue differing very little from green; while the blue 

 of the spectrum appears to contain as much violet as green : and it is for this 

 reason that red and blue usually make a purple, deriving its hue from the pre- 

 dominance of the violet. 



It would be possible to exhibit at once to the eye the combinations of any 

 three colours in all imaginable varieties. Two of them might be laid down 

 on a revolving surface, in the form of triangles placed in opposite directions, 

 and the third on projections perpendicular to the surface, which, 

 while the eye remained at rest in any one point, obliquely situated, would 



