482 SCIENCE PROGRESS 



portions of the retina requiring a stronger stimulus than the 

 more developed portions. 



Much of the experimental work on colour vision has been 

 done with bits of coloured paper and impure colours, which is 

 similar to conducting a chemical analysis with impure chemicals; 

 many of the results are due to stray light and entirely different 

 results are obtained when pure spectral light is employed. 



X. Facts supporting the Theory of the Relative Functions 

 of the Rods and Cones of the Retina (29, 30, 31) 



There is not a single fact pointing to the view that the rods 

 are percipient elements. The attribution of this function to 

 them is the purest assumption, and recent writers are now recog- 

 nising that this is the case. On the other hand on a photo-chemical 

 theory of vision an elaborate nervous mechanism is required to 

 repair and regulate the photograph in various physical conditions 

 (32). This function is performed by the rods, which contain a 

 photo-chemical substance, the visual purple, which is not present 

 in the cones ; they liberate the visual purple into the fluid 

 surrounding the cones, and the decomposition of this photo- 

 chemical fluid by light stimulates the ends of the cones, and the 

 visual impulses are started. 



From an anatomical point of view it seems impossible that 

 the rods could be percipient elements. The reader should 

 examine some recent reproductions of microscopic specimens of 

 the retina. It will be noticed that the rods terminate in rounded 

 knobs, many of which are in connection with one neuron. It 

 will also be noticed that transverse neurons connect many 

 groups of rods, and this transverse neuron is only indirectly 

 connected with the ganglion cell leading to the fibre of the 

 optic nerve. Now, in order that any percipient element may be 

 able to act as such, the anatomical paths must be different, but 

 how can the rods act as percipient elements when a large group 

 terminate in the same path ? It will be noticed that this 

 anatomical arrangement is perfect from the point of view that 

 the rods regulate the distribution of the visual purple into the 

 liquid surrounding the cones. 



The visual purple is found in the rods and not in the cones, 

 but if the external surface of the retina, of a monkey which has 

 been kept for forty-eight hours in a dark room, be examined, the 

 visual purple will be found between and not in the cones (33). 



