CHAP. XI.] PERIPHERAL NERVOUS END-ORGANS. 467 



fact that a temperature of - 13 C. does not materially impede the 

 bleaching by light. 



Action of Caustic alkalies, acids, alcohols, ether, and chloro- 



various che- form decolourize the retinae of recently killed frogs. On 

 micai agents the other hand, many agents whose activity niight 

 Purpie Ual kave k een presumed upon, such as ammonia, alum, 



the process of putrefaction, trypsin, are ineffectual in 

 changing the visual purple. 



When de- ^ n describing the effect of various agents upon the 



prived of visual purple that body has usually been under con- 



water the ditions which presupposed the presence of water. If, 



Visual Purple however, water be withdrawn from the structure or 



su kstance coloured with visual purple, though that 

 substance continues to be affected by sunlight, the time 

 during which the light must act is enormously increased. 



optograms ^ G ^ act ^ a * ^ ie ^ V ' IU S retina possesses a colouring 



matter which is decomposed by light led Kiihne very 

 early to enquire whether it was possible, under certain circumstances, 

 to obtain actual images on the retina, corresponding to objects which 

 have been looked at. After his first experiments Ktihne endeavoured 

 to observe, on the retinae of rabbits, bleached spots corresponding to 

 the images of external objects, but his endeavours failed. In the 

 course of his researches Kiihne discovered the remarkable fact which 

 will be described in the succeeding section, viz. that there exist 

 within the retina agents which are concerned in the restoration 

 of the visual purple. Taking for granted that such agent or agents 

 exist, it will follow that, in order to obtain on the retina a picture 

 of external objects, the effect of the light would have to be so 

 prolonged or so intense as to destroy the balance between the 

 destruction of the visual purple and the power possessed by certain 

 retinal elements to restore it. 



Kiihne took a coloured rabbit and fixed its head and one of its 

 eye-balls at a distance of one metre and a half from an opening 

 thirty centimetres square in a window shutter. The head was 

 covered for five minutes with a black cloth, and then exposed for 

 three minutes to a somewhat cloudy mid-day sky. The animal was 

 then instantly decapitated; the eye-ball which had been exposed was 

 rapidly extirpated by the aid of yellow light, then opened, and 

 instantly plunged in a 5 per cent, solution of alum. Two minutes 

 after death, the second eye-ball, without removal from the head, was 

 subjected to exactly the same processes as the first, viz. to a similar 

 exposure to the same object, then extirpated. On the following 

 morning the milk-white and now toughened retinae of both eyes were 

 carefully isolated, separated from . the optic nerve, and turned ; 

 they then exhibited on a beautiful rose-red ground a nearly square 

 sharp image with sharply-defined edges; the image in the first eye 



302 



