106 HYPOTHESES OF VISION. [MEMOIR VI. 



eventually to a deep mahogany red, and chemically to 

 throw it from an active into an inactive state. As amor- 

 phous phosphorus, otherwise prepared, it ceases to shine 

 in the dark. In the experiments now alluded to, it ap- 

 peared that this reddening takes place in the indigo and 

 violet spaces, so that the fixed lines known by spectro- 

 scopists as those about H were beautifully depicted. 

 Now some physiologists have supposed that nerve ves- 

 icle tissue owes its property to the presence of uuox- 

 idized phosphorus, but if the principles we are contem- 

 plating be correct, and this were the case, the most brill- 

 iant ray in the spectrum should be the indigo, and not 

 the yellow. Therefore, if vision be performed by chem- 

 ical change in the substance of the retina, it is carbon 

 and not phosphorus that is concerned. 



If we admit that during the act of vision the retina, 

 as a structure with a carbon nucleus, undergoes meta- 

 morphosis, the principles of photo-chemistry would lead 

 us to expect that the yellow must be the brightest ray, 

 and a harmony is thus established between this and 

 other functional changes in the body. We also perceive 

 the significance of certain structures of the eye which 

 otherwise would appear to be without meaning. The 

 rapid retrograde metamorphosis which must be taking 

 place in the retina involves the provision of some means 

 for moving away the wasted products and of supplying 

 nutrition with the utmost quickness. And this is the 

 office discharged by the choroid. 



But such removals and supplies require time. Time, 

 therefore, enters as an element in the visual operation. 

 Sis;ht commences instantaneouslv, but the ima^e of an 



O / 7 O 



object may be seen long after the reality has disap- 

 peared. This instantaneous commencement of a retinal 

 impression may be very strikingly illustrated. The 

 spark of a Leyden-jar, though it does not last, as is af- 



