ANIMAL AND VEGETABLE 227 



the rods and cones, which we are told are the structures 

 directly concerned with vision. 



In the conversion of rays of light into waves of neuro- 

 electricity delays which would reduce the rate of trans- 

 mission to the normal velocity of nervous impulse would 

 most certainly occur at the synapses, and quite apart from 

 physiological research we can be reasonably sure that the 

 impulses to which vision is due do not travel at anything 

 like the rate at which electro-magnetic waves are pro- 

 pagated. Halliburton says : " The duration of the sensa- 

 tion produced by a luminous impression on the retina is 

 always greater than that of the impression which produces 

 it. However brief the luminous impression, the effect on 

 the retina always lasts for about one-eighth of a second.*' 

 That is, in perfect harmony with an electrical impulse, 

 which, as we have seen (p. 160), always takes longer to 

 leave the circuit than it did to enter it, but it is not in 

 harmony with the theory that impulses are conveyed to the 

 brain at a velocity of 300,000,000 metres instead of 120 

 metres per second. In the one-eighth of a second during 

 which the retina retains the impression no fewer than 

 1,500,000,000 impulses would be produced by the direct 

 vibrations of electrons, and they would continue to arrive 

 at the same speed while vision lasted. 



Some further arguments in favour of the theory I have 

 advanced may, however, be adduced. 



I have said that, in my opinion, the optic, like the 

 auditory, nerves and we must include their processes 

 are " closed " circuits. Halliburton states that the 

 retina " possesses a store of potential energy which the 

 stimulus serves to fire off." That is understandable in a 

 closed, but not in an open, circuit. 



44 Nothing is known about the yellow pigment of the 

 yellow spot," but a 4t change produced by the action of 

 light upon the retina is the movement of the pigment cell*. 



