104 HYPOTHESES OF VISION. [MEMOIR VI. 



ficulties. Heat suffers conduction. If this black pig- 

 ment officiated as a transformer of light rays into heat 

 by producing extinction, there must unavoidably be a 

 lateral spread from the boundaries of warm to cooler 

 spaces, the edges of images must be nebulous and with- 

 out sharpness of contour. Moreover, there is reason to 

 believe that the visual apparatus cannot take cognizance 

 of heat merely as such. Calorific rays reach the black 

 pigment and raise its temperature without the retina 

 being affected. 



Such considerations seem, therefore, to exclude the cal- 

 orific hypothesis, and prepare us for an examination of 

 the chemical. 



SECOND, of the chemical hypothesis of vision. Numer- 

 ous discoveries made of late years in relation to the 

 chemical action of light put us in possession of many 

 facts having a bearing on this hypothesis. A majority 

 of compound substances, both inorganic and organic, suf- 

 fer chemical modifications when exposed to the access of 

 light, and, what is very significant, these changes are oc- 

 casioned by definite classes of rays. One substance finds 

 its maximum of action in the violet region, another in 

 the yellow, another in the red. The effect in every in- 

 stance grades off towards the less and more refrangible 

 spaces respectively. 



In these actions of decomposition there is nothing like 

 lateral spreading, nothing answering to conduction. No 

 better proof of this is necessary than the exquisite sharp- 

 ness of photographic pictures a sharpness only limited 

 by the optical imperfections of the lens with which they 

 are made. The molecules on which the light falls are 

 the only ones that experience change ; there is no propa- 

 gation of effect from part to part an important particu- 

 lar, because it is what we observe in the case of sight. 



The retina, the nervous expansion of the eye, is so con- 



