SENSE OF SIGHT. 447 



these shadows, is separated from these vessels by a distance 

 which is exactly equal to that which by microscopical men- 

 surations of sections of the retina exists between the layer in 

 which the vessels are situated and Jacob's membrane ; the 

 sensitive layer of the retina consists, therefore, of the layer 

 of rods and cones. 



Now that we have seen that the seat of sensibility is in 

 one of the layers of the retina, the extreme posterior layer, 

 we can no longer be content with the empty formula that the 

 retina is a screen / nor consider it sufficient to trace the prog- 

 ress of the light through the media of the eye to the surface 

 of the sphere of the retina. The rays of light pass through 

 all the layers of the retina without making any impression, 

 as has been shown, first by Rouget, and since by Desmoulins ; 

 on reaching the part where they come in contact with the 

 rods and the choroid tunic, they are reflected ; and, as the 

 optical centre obviously coincides with the centre of the ret- 

 inal curve, the reflexion naturally takes place in the direction 

 of the axis of the rods and cones. The external segments of 

 the cones and the rods, however, as has been proved by 

 Schultze, 1 consist of small lamellae, placed one upon the other ; 

 and these, on account of their structure and optical proper- 

 ties, cannot be considered as being susceptible to light, but 

 only as organs which serve to modify the light. It is now 

 generally supposed that at the instant when the light reflected 

 by the choroid mirror (Rouget) passes back through the 

 retina, a peculiar transformation takes place in these organs, 

 constituting a sort of necessary intermediation between the 

 physical phenomenon of light, and the physiological phenom- 

 enon of nervous excitation. Without exactly defining the in- 

 timate nature of the process which here occurs, we may con- 

 sider it as a transformation of force / in other words, the 

 luminous movement (vibrations of the ether) is changed into a 

 nervous movement (nervous vibration. See p. 29). The ex- 

 ternal parts of the rods and cones are, in themselves, insensi- 

 tive to luminous impressions, but they form organs of trans- 

 formation of the waves of light, that is, special agents by 

 which the light is transmitted to the optic nerve. 



The internal segments of the rods and cones are then the 

 organs which are essentially susceptible to light. The dif- 

 ferences of function, which correspond with the differences 



1 See a resume of these researches. Duval, " Structure et 

 Usage de la Retine." Paris, 1873. These d'Agreg. ' 



