THE EYE AS A SENSORY APPARATUS 237 



It is said that some animals, such as snakes, have no rods in 

 their retinas, while the retinas of others of nocturnal habits, such 

 as owls, consist exclusively of rods. 



Visual Purple. If a perfectly fresh retina be excised rapidly, 

 its outer layers will be found of a rich purple color. In daylight 

 this rapidly bleaches, but in the dark persists even when putre- 

 faction has set in. In pure yellow light it also remains unbleached 

 a long time, but in other lights disappears at different rates. If a 

 rabbit's eye be fixed immovably and exposed so that an image 

 of a window is focussed on the same part of its retina for some 

 time, and then the eye be rapidly excised in the dark and placed 

 in solution of potash alum, a colorless image of the window is 

 found on the retina, surrounded by the visual purple of the rest 

 which is, through the alum, fixed or rendered incapable of change 

 by light. Photographs, or optograms, are thus obtained which 

 differ from the photographer's only in the nature of the chemical 

 substances and processes involved. Both depend on the produc- 

 tion of a chemical reaction by light. If the eye be not rapidly ex- 

 cised and put in the alum after its exposure, the optogram will 

 disappear; the vision purple being rapidly regenerated at the 

 bleached part. This reproduction of it is due mainly to the cells 

 of the pigmentary layer of the retina, which in living eyes ex- 

 posed to light thrust long processes between the rods and cones. 

 Portion of frogs' retinas raised from this, bleach more rapidly 

 than those left in contact with it, but become soon purple again 

 if let fall back upon the pigment-cells. 



The visual purple, as stated previously, occurs only in the outer 

 segments of the rods. Whatever function it has is probably con- 

 nected, therefore, with their special property of reacting to feeble 

 lights. The nature of its function is, further than this, unknown. 



The Duration of Luminous Sensations. This is greater than 

 that of the stimulus, a fact taken advantage of in making fire- 

 works: an ascending rocket produces the sensation of a trail of 

 light extending far behind the position of the bright part of the 

 rocket itself at the moment, because the sensation aroused by it 

 in a lower part of its course still persists. So, shooting stars ap- 

 pear to have luminous tails behind them. By rotating rapidly 

 before the eye a disk with alternate white and black sectors we 

 get for each point of the retina on which a part of its image falls, 



