ON LIGHT. 249 



inhabits dark caves, and whose dehcate skin is evidently 

 and painfully affected by the light ; but to convey the 

 perception of form, a picture must be produced, and in 

 its own peculiar manner. 



(33.) We are now prepared to understand the mode 

 in which colour originates. This, to tlie ancients, was 

 always a mystery. The light of the sun, and of ordinary 

 daylight, which is only that of the sun dispersed and 

 reflected backwards and forwards among the clouds, is 

 white, or nearly so. Nevertheless, when we look through 

 a red glass, or view a green leaf, it conveys to the mind 

 the perception of those colours. How is this ? If it be 

 by light only that we see, and if that light convey to us 

 absolutely none of the materiah elements of the bodies 

 from which we receive it, how comes it that it excites in 

 us such various and perfectly distinct sen^^ations? The 

 light itself must have either acquired or parted with 

 something in its passage through or reflexion from the 

 coloured body. Supposing, for instance, light to be a 

 substance ; it may have taken up some excessively 

 minute portion of the object and introduced it to the 

 direct contact of our nerves. In that case the sense of 

 colour would be assimilated to those of taste or smell. 

 Or it may have undergone analysis^ and colour would 

 then arise from a dejiciejicy of something existing in the 

 sun's light, and the relative I'edimdancy of some other 

 portion. In this view, light would be regarded, not as a 

 simple, but a compound substance, or a mixture of so 

 many simple ones as would suffice to explain all the 

 observed differences of tint. On the other hand, if light 



