4G2 VISUAL PURPLE OR RHODOPSIN. [BOOK I. 



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. 



The wonderfully suggestive nature of Boll's discovery led Kiihne 

 to repeat his observations 1 . Whilst generally confirming the funda- 

 mental statement of Boll, Kiihne at once was able to correct and 

 amplify Boll's account. In the first place, relying implicitly upon the 

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

 retinae of animals (frogs and rabbits) 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 

 bleaching takes place so slowly in gas-light 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 disappear in from twenty-four to twenty-eight hours 

 even though decomposition has set in. These first observations of 

 Kiihne on the vision-purple (Sehpurpur), as he termed it, whilst they 

 shewed that the disappearance of the colour is not, as Boll had assert- 

 ed, a necessary concomitant of death, removed many of the difficul- 

 ties 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 almost at once to discover the conditions necessary 

 to the destruction of the vision-purple, as well as the most important 

 facts relating to its restoration or removal. Since then the investi- 

 gation of the retinal pigments and of photo-chemical processes in the 

 eye ha,ve formed the subject of continuous and successful studies on 

 the part of Kiihne and his pupils, and it is to them that we owe all 

 the important facts relating to this fascinating subject 2 . 



1 Kiihne, "Zur Photochemie der Netzhaut." Gelesen in der Sitzung des Natur- 

 historisch-medicinischen Yereins zu Heidelberg, den 5 Jan. 1877. 



2 The following is a list (in the order of publication) of the researches of Kiihne and 

 his pupils on the retinal pigments and photochemical processes hi the retina which have 

 appeared in the Untersuchungen aus dem physiologischen Institute der Universitdt 

 Heidelberg. 



(1) Kiihne, " Zur Photochemie der Netzhaut." (2 Abdruck.) UntersucJiung. Vol. i. 

 Part i. 



(2) Kiihne, "Ueber den Sehpurpur." Ibid. 



(3) Kiihne, "Ueber die Verbreitung des Sehpurpurs im menschlichen Auge." 

 UntersucJiung. Vol. i. Part ii. p. 105. 



(4) Kiihne, "Weitere Beobachtungen iiber den Sehpurpur des Menschen." Ibid. 

 p. 109. 



(5) Kiihne, ' 'Das Sehen ohne Sehpurpur." Ibid. p. 119. 



(6) Ewald u. Kiihne, "Untersuchungen iiber den Sehpurpur." Ibid. p. 139. 



(7) Kiihne, "Ueber die Darstellung von Optogrammen im Froschauge." Ibid. 

 Vol. i. Part iii. p. 225. 



(8) Kiihne, "Eine Beobachtung iiber das Leuchten der Insectenaugen. " Ibid. 

 p. 242. 



(9) Ewald u. Kiihne, "Untersuchungen iiber den Sehpurpur." (Fortsetzung. ) 

 Ibid. p. 248. 



(10) Kiihne, "Ueber lichtbestandige Farben der Netzhaut." Ibid. Vol. i. Part 

 iv. p. 341. 



