vertical lines ; muscce volitantes produced by dark specks which 

 float in the transparent humour which fills up the space between 

 the lens and retina ; and the star-shaped appearance of a bright 

 light, caused by the radiating structure of the lens, &c. 



Of the parts of the eye which receive the image, the most 

 important structures are believed to be the rods and cones, spread 

 like a very fine mosaic over the inner surface of the retina. Two 

 points looked at, at the same moment, can only be seen as distinct 

 from each other when the images fall on separate cones, but we 

 have little cause of complaint in this limitation of our powers, when 

 we learn that there are nearly a million of these cones in the 

 hundredth part of a sqiiare inch. The portion of the retina where 

 the bundle of optic nerves enters, is totally insensitive to light, and 

 forms a " blind spot " of about one-twelfth of an inch in diameter, 

 but the defect is not generally noticed in our vision, as the portion 

 of the image lost on the blind spot of one eye does not fall on the 

 blind spot of the other eye. For accurate observation, only a very 

 small part of the retina is used. This portion, known as the 

 " yellow spot," lies near the blind spot, and is also about one- 

 twelfth of an inch in diameter. Almost all the elements of the 

 retina, except the cones, are absent from this spot ; even the blood 

 vessels, which are present everywhere else, form a ring round this 

 "holy of holies." Man shares with the monkey alone the 

 possession of this peculiar structure. 



A purple red fluid formed on the rods and cones, is bleached 

 by the action of light (just as the silver salts of the photographer's 

 negative are acted upon in the camera), and these bleached images 

 of external objects have been observed and rendered permanent by 

 a solution of alum, in the case of rabbits decapitated immediately 

 after exposure to a strong light : a recent attempt to present 

 Helmholtz with a photograph of himself obtained in this way was, 

 however, a failure. 



Young's hypothesis, that to every cone are attached three 

 nerve fibres, which, respectively, convey the sensation of red, green, 

 and violet light, was explained. Thus, the impression of white 

 light is believed to be obtained when all the fibres are acted on in 

 due proportion, and that of colour when there is an unequal 



