PHYSIOLOGY OP VISION AND SENSATION OF COLOUR. 231 



half-an-hour, no trace of the original colour could ever be 

 found, and the dying retina did not show a yellowish, but a 

 pure white satiny lustre. Exactly the same experiments 

 were simultaneously performed at a window of the laboratory 

 with a northernly aspect, with the result that diffused daylight 

 requires double or triple the time of direct sunlight to de- 

 colourise the retina completely. In another series of expe- 

 riments, finally, a dozen frogs which had been exposed to 

 direct sunlight for more than an hour were returned to 

 absolute darkness and examined in succession. The first 

 traces of returning redness were never found in these frogs 

 before the lapse of one hour, and were very weak for the 

 most part, even after an hour and a half; after two hours a 

 very intense colouring was usually again present. 



II. I repeated the experiments on monochromatic dazzling, 

 and must remark that in these experiments I did not yet use 

 truly monochromatic light, but was obliged to employ the 

 same more or less faulty glasses which had served for the 

 first experiments with light of medium intensity. These 

 experiments yielded the following results : 1. The operation 

 of the most intense red and yellow light, even when con- 

 tinued for several hours, allowed the red colour of the retina 

 to be preserved, but with the difference (scarcely noticeable 

 in experiments with light of medium intensity) that it be- 

 came in red light still more intense and darker, of a " red- 

 brown" or nearly brown tint, while in yellow light it ap- 

 peared clearer and lighter, nearly " rosy." The ground- 

 colour of the retina lies midway between these two extreme 

 shades of colour. 2. The dazzling by green light after short 

 duration gives the same results as with the same light of 

 medium intensity. If the dazzling be continued longer than 

 two hours, the colour of the retina becomes altered in the 

 same way as by the action of blue light of medium intensity, 

 i. e., it becomes violet. In still longer dazzling this violet 

 becomes paler and paler and at last the retina appears nearly 

 entirely colourless. 3. Dazzling by blue and violet light of 

 short duration gives quite the same results as the same light 

 of medium intensity. If the dazzling be continued for two 

 hours or more, the violet colour of the retina becomes con- 

 siderably paler and finally the retina becomes quite colourless, 

 as after the action of white light. 4. In these experiments 

 a phenomenon which was also noticed in earlier experiments 

 came out very distinctly : hand in hand with the progressive 

 decolourizing of tlie retina, there takes place a peculiar alte- 

 ration in the consistency of the layer of rods and in the 

 retinal pigment. While in eyes that have been kept in 



