290 Prof. J. Joly on a 



absorption of the visual purple or rhodopsin (see (8)). 

 Further, Ktihue states that the maximum spectral absorption 

 of this substance is at that part of the spectrum which seems 

 brightest to the eye, and \\ hich is most active in bleaching 

 the rhodopsin. It seems therefore very probable that all 

 over the retina it is. this substance which forms the inter- 

 mediary between the light and the nerve ; translating the 

 quanta of light-energy into nerve-stimulus. 



It is an important feature of the theory herein advocated 

 that the sensitiser in the case of the cones — as the organs of 

 colour vision — should lie outside the cone and should not exist 

 within it as in the case of the rod. That it does not exist 

 within the cones is agreed by all observers.. Dr. Edridge 

 Green (' The Physiology of Vision/ 1920, p. 43) claims to 

 have seen the unbleached rhodopsin between but not in 

 the cones of the fovea. He states that when the retina 

 was first examined the fovea was the reddest part of the- 

 w T hole retina. He also calls attention to a confirmatory 

 observation of Kiihne's in the case of a shark's retina. 

 Mr. J. Herbert Parsons more recently points out that 

 Hering had recognized such a distribution of rhodopsin 

 as possible (Brit, J. Ophthal., July 1920)*. 



I assume that in the case of the rods the sensitiser is- 

 operative within the nerve. In the case of the cones it 

 is operative from without, 



(z) The belief is gaining ground that photo-chemical and 

 photo-electric processes are fundamentally alike (Lewis,. 

 Physical Chemistry, iii. p. 134). In the case of photo- 

 graphic actions the view that the movement of electrons 

 within the light-sensitive film is responsible for the phe- 

 nomena observed is supported by facts regarding the 

 various modes in which the plate may be stimulated and 

 by the formation of the latent image. ~No other theoretical 

 basis affords so general an explanation of the effects of the 

 light, (See ' Photo-Electricity/ by H. Stanley Allen : 

 Longmans Green, 1913.) The range of photographic 

 " vision " may be controlled by the use of a sensitiser. 

 This substance is one which absorbs vigorously in one 

 or more special regions of the spectrum. It sensitises 

 the plate to the same range of wave-lengths as it absorbs. 

 The photographic sensitisers are rich in light-absorbing 

 molecules : — chromogens. They are photo-electric : emitting 

 electrons over the range of frequencies which they absorb. 



* Dr. Edridge Gieen believes that vision is due to photo-chemical 

 action progressing in the rhodopsin surrounding the cones. He considers 

 that the reds are not percipient. 



