156 PROFESSOR FRANZ COLL. 



physicist and physiologist^ that I think an abstract of it Avill 

 be acceptable to the readers of ' Nature.' 



A short time since. Boll (a pupil of Max Schultz and Du 

 Bois-Reymond, who now occupies the chair of physiology in 

 Rome) communicated to the Berlin Academy the remarkable 

 fact that the external layer of the retina, i. e. the layer of rods 

 and cones, possesses in all living animals a purple colour. 

 During life, according to Boll, the peculiar colour of the retina 

 is perpetually being destroyed by the light which penetrates 

 the eye ; darkness, however, restores the colour, which 

 vanishes for ever almost immediately after death.i 



The wonderfully suggestive nature of Boll's discovery led 

 Kiihne to repeat his observations ; in doing so, whilst he has 

 confirmed the fundamental statement of Boll, he has ascer- 

 tained a number of new facts of great interest. 



Kiihne's observations were made on the retinae of frogs 

 and rabbits. In the first place, implicitly relying upon the 

 statements of Boll, he examined, as soon as possible after 

 death, the retinae of animals which had been kept for some 

 time in darkness. He soon found that the beautiful purple 

 colour persists after death if the retina be not exposed to 

 light ; that the bleeching takes place so slowly in gaslight, 

 that by its aid the retina can be prepared and the changes 

 in its tint deliberately watched ; that when illuminated with 

 monochromatic sodium light the purple colour does not dis- 

 appear in from twenty-four to twenty-eight hours, even though 

 decomposition have set in. 



The first observations of Kiihne on the vision-purple 

 {Sehpurpur), as he terms it, whilst they showed that the 

 disappearance of the colour is not, as Boll has asserted, a 

 necessary concomitant of death, removed many of the diffi- 

 culties which stood in the way of a careful investigation. 

 Carrying out his preparations in a dark chamber illuminated 

 by a sodium flame, Kiihne was able to discover the conditions 

 necessary to the destruction of the vision-purple as well as 

 some facts relating to its restoration or renewal. 



As long as the purple retina is kept in the dark or is 

 illuminated only by yellow rays, it may be dried upon a glass 

 plate without the tint changing ; the colour is not destroyed 

 by strong solution of ammonia, by saturated solution of com- 

 mon salt, or by maceration in glycerine for twenty-four hours. 



* This account of Boll's researches is taken from Kiihne's paper. The 

 latest number of the ' Monatsberichte' of the Berlin Academy whicli has 

 yet reached Manchester, which includes the Proceedings for September 

 and November, does not contain Boll's communication, which is of later 

 date (November 12). 



