MEMOIR VI.] HYPOTHESES OF VISION. 



sti tuted that a maximum effect upon it is occasioned by 

 the yellow ray, the action declining on one side to the 

 red, and on the other towards the violet, and ceasing 

 at the extremes of those rays. For this reason, when a 

 solar spectrum is examined by the eye, the yellow is the 

 most brilliant space, there being a decline in intensity 

 from it to the two extremes. 



In my experiments on the decomposition of carbonic 

 acid by plants in the sunlight, to be described hereafter 

 in these memoirs, the maximum of action was found to 

 be in the yellow, with a gradation of effect towards the 

 red and violet ends of the spectrum respectively. From 

 this it would appear that a relation exists between light 

 and compounds having a carbon nucleus, answering to 

 that observed in the case of the retina of the eye. Such 

 a relation is very well illustrated in the case of other 

 chemical elements, as silver, a metal which is the basis 

 of all ordinary photographic preparations. The ray of 

 maximum action is in the indigo space. Objects viewed 

 by a retina having a silver sensitive nucleus would pre- 

 sent an appearance altogether unlike that they would 

 offer to a carbon nucleus. The order of brilliancy in 

 the lights would be no longer the same. The red and 

 yellow parts of objects would be black, that is to say, 

 invisible, and other rays beyond the violet would come 

 into view. 



Among experiments I have made on this subject, 

 there is one of much physiological interest. The ele- 

 ment phosphorus finds its maximum impression in the 

 more refrangible portion of the spectrum, in that respect 

 resembling silver. Upon a portion of translucent phos- 

 phorus, enclosed out of contact of air in a flattened glass 

 tube, into which it had been drawn while melted, and 

 then suffered to solidify, a solar spectrum was cast. The 

 effect of light upon this kind of phosphorus is to turn it 



