2 PHOTOCHEMISTRY OF VISUAL PURPLE. I 



In terms of such an hypothesis of photoreception it would be a 

 distinct step forward to be able to identify the three components of 

 the reaction, and to discover their exact chemical interrelations. At 

 present no definite statement is possible in this connection. However, 

 dark adaptation is decidedly a phenomenon of dim vision. According 

 to the Duplicity Theory, dim vision is associated with the rods of 

 the retina. The rods contain a photosensitive substance known as 

 visual purple. It is therefore possible that dim vision and, more par- 

 ticularly, dark adaptation are in some way conditioned by the proper- 

 ties of visual purple. 



The association of visual purple Avith dim vision rests on more than 

 the mere possibility just stated. A comparison of the threshold of 

 visual sensibility at different wave-lengths with the spectral absorp- 

 tion of visual purple and its photochemical sensitivity has shown that 

 these three sets of data resemble each other to a striking degree (Tren- 

 delenburg, 1911). It is similarly with the relation between dark 

 adaptation and visual purple. A retina exposed to light is bleached. 

 Placed in the dark, the intact eye or the retina alone with its pigment 

 layer will regenerate the purple color. Under similar conditions the 

 lixdng eye becomes dark-adapted. The presumption is therefore that 

 these phenomena are in some way correlated. 



Our judgment in this matter must, however, be withheld, because 

 no quantitative data are available with regard to the photochemistry 

 of visual purple. Ever since Boll's (1876) discovery of this substance, 

 and Kiihne's classic qualitative investigations with it (Ewald and 

 Kiihne, 1878), there have been but a few scattered observations made 

 with visual purple. A notable exception is the work of Trendelenburg 

 (1904). It, however, has been barely a beginning. 



It has therefore been necessary to investigate the visual purple 

 mechanism from the modern point of view of photochemistry, and to 

 attempt a complete analysis of the reactions concerned. The present 

 paper is the first of a series in which the results of this investigation 

 are to be presented. As a product of these researches, I hope to be 

 able first, to decide whether visual purple is really intimately con- 

 cerned with dark adaptation and the reception of dim lights, and, 

 second, if it is so concerned, to determine precisely the way in which 

 this is brought about. 



