180 SATURDAY LECTURES. 



ill molecular structure, is also different for the different 

 colors. Moreover, the effect of this change in molecular 

 structure is conve3'ed to great distances through other 

 media, and there makes itself manifest under the same or 

 some other form. 



It is thus evident that in the inorganic world the light 

 rays do produce different affects, as manifested by molecular 

 change, according to their different wave-lengths. We have 

 only to apply this law to the action of light on the retina 

 to have a theor}" of colors at once simple, and strictly in 

 accordance with known })rinciples, and without the necessity 

 of inventing new tissues or extraordinary reactions of tissues 

 to light. In this theory variation in sensation will have its 

 basis, not in complexity of tissue, but solel}' in the varying 

 action of the affecting agent. 



In accordance with this hypothesis we have onl}- to sup- 

 pose the retina to be a body whose molecular structure is so 

 delicately fashioned as to allow it to respond promptly to 

 the slightest variation in the wave-lengths of light raj'^s. 

 We know that up to a certain point we can discriminate 

 between fine shades of intensity of light. The educated eye 

 of an artist can detect as minute differences in shading as 

 he can in colors. This we must suppose is due to the judg- 

 ment which has been educated to discriminate between 

 small differences as to quantity, and there can be no reason 

 for not supposing that we can learn in like manner to dis- 

 tinguish between small differences in qualit}'' as manifested 

 in the slight alteration in the molecular arrangement of the 

 retina brought about by the different colors. Eveiy impres- 

 sion, therefore, produced by each color will be conve3'ed in- 

 dependently of other impressions to the brain, and will there 

 be turned into a distinct and individual sensation. But 

 even in this theory there is no necessity for supposing that 

 all the color sensations are primary. We can have here, as 

 in the other theories, a mixture of sensations in the produc- 

 tion of some colors, and the numerous combinations which 

 are known under the name of secondary colors. Purple, 

 for example, will be the sensation resulting from a combi- 



