VISION 381 



pigment, and shorten in bright light. These movements may 

 merely indicate that the cones require a backing of pigment, 

 but it would seem more probable that, like the rods, they 

 absorb a substance which is sensitive to light, although we 

 cannot recognize it by its colour. 



The responsiveness of the rods to light is due to visual 

 purple. As every lady is aware, colours, especially mauves 

 and lilacs, are bleached by light. The chemical change affected 

 by light in the colour of the outer limbs of the rods is the 

 stimulant which originates impulses in the nerve-fibres con- 

 nected with them, and it is generally believed that cones the 

 more highly specialized sensory cells are stimulated in the 

 same way. Visual purple is particularly abundant in all 

 animals that range at night, with the exception of the bat. 

 But its absence in the bat does not militate against the theory 

 that it is the cause of night- vision, for it has been shown that 

 a blind bat flies with almost as much freedom, and avoids 

 obstacles even threads stretched across the room with as 

 much skill as one that can see. It is guided by the bristles 

 of its cheek. So, too, is the cat, which has the reputation of 

 being able to see in the dark. Undoubtedly a cat's eye is an 

 exceptionally efficient organ in dim light, just as it is excep- 

 tionally sensitive to sunshine it is provided with an iris which 

 contracts the pupil almost to a pinhole but the cat trusts 

 to the bristles of its cheek for information regarding the things 

 which block its path. 



Most of the peculiarities which distinguish the reactions of 

 the eye from those of other sense-organs can be explained by 

 its mode of stimulation the initiation of a nerve-current by 

 a chemical change. No stimulus, if sufficiently strong, can 

 be too brief. The retina reacts to an electric spark in the same 

 way as a photographic plate ; but, unlike the plate, the retina 

 is restored to its previous condition of sensitiveness in about 

 one-tenth of a second. A visual sensation lasts about one-tenth 

 of a second. This prolongation of the sensation is, however, a 

 mental, not a retinal, effect. The mind continues to see an 

 object which has been illuminated by a flash until the retina 

 is again in a condition to send brainwards a second impulse. 

 Were our sensations coincident in duration with the stimulation 

 of our sense-organs, we should live in a flickering cinemato- 



