466 VISUAL PURPLE OR RHODOPSIN. [BOOK I: 



entire beam of white light is by far the best transformer of the visual 

 purple superior to light of any particular wave-length. The less 

 refrangible dark rays at the red end of the spectrum do not bleach 

 the visual purple. Whether the actinic rays at the violet end are 

 capable of slowly bleaching is not yet ascertained, though it is certain 

 that if possessed of activity it is almost immeasurably weaker than 

 that exerted by the coloured rays. Of the visible rays of the spectrum 

 those bleach the visual purple most freely and quickly which the 

 visual purple in solution most effectually quenches. Thus the order 

 of activity in the bleaching of the purple is as follows : yellowish- 

 green, green, blue, green-yellow, yellow, violet, orange and red. 

 Between yellow-green and yellow the time of bleaching is con- 

 siderable ; it is less between yellow-green and green up to blue. 



But intensity of light, or the quantity on the unit of surface, has 

 an influence which renders the above classification very general and 

 bespeaks for it some latitude. We may in connection with this 

 subject draw attention to a practical point, which has already been 

 referred to incidentally. The inconvenience of the preparation of the 

 visual purple in the dark or in a dim twilight may be met by using 

 a monochromatic light of slight decomposing powers. A glance at 

 the previously mentioned orders of decomposing-activity of light of 

 different colour will convince us that red would be the best light for 

 the preparation of visual purple, were it not that in red light it is 

 impossible to detect and avoid blood stains. But yellow light from a 

 sodium flame, which takes about two hours to bleach a frog's retina, 

 is a useful substitute. 



Although rays from different regions of the spectrum differ in 

 their powers of transforming visual purple, yet no visible ray fails to 

 bleach it if the exposure be prolonged enough. Further, the rays 

 differ among themselves in the rate at which they convert and bleach 

 the retinal colours. White light, to which we refer as a standard, 

 brings about the following transitions from purple : red-purple, pure 

 red, orange, yellow, chamois-yellow to no colour whatever. 



On the other hand, the red rays of the spectrum produce a change 

 through pure red and orange to the palest yellow, whilst taking an 

 extraordinarily long time to do so. And the rays from the opposite 

 end of the spectrum cause the purple to merge into a final stage which 

 is not yellow but bright red or lilac. In other words, as the wave- 

 lengths diminish less and less visual yellow is produced ; or, perhaps, 

 the yellow which is produced is bleached as quickly, or more quickly, 

 than the still unaltered purple. 



Influence of Light is not the only agent which affects the visual 



temperature purple. When retinae are exposed to temperatures 

 upon the varying from about 50 C. to 76 C. the colour fades with 



?le> a rapidity which increases with the temperature. At 

 52 53 in the absence of light it takes some hours to disappear, at 76 

 it disappears instantly. That a low temperature does not interfere 

 with the action of light upon the purple-stained rods is shewn by the 



