228 FRANZ BOLL. 



yellowish-green as far as E, and then absorbs all the re- 

 maining green, and the blue and violet to the end of the 

 spectrum. 



Macroscopically and microscopically the retina, exposed to 

 yellow light, behaves in the same way as after the operation 

 of red light. 



3. Green Light. 



The green glass absorbs completely the red and orange to 

 D, transmits yellow and green from D to b, absorbs the 

 greater part of the dark green from b to F, and completely 

 the rest of the spectrum from F. 



Macroscopically the ground-colour of the retina appears 

 changed to " purple red." Under the microscope the red 

 rods appear of the same colour; as they pale they pass into 

 a beautiful rose tint. The green rods show the same bril- 

 liant tint as after the operation of red and yellow light. 

 Their number, compared with that in retinae kept in the 

 dark and in red and yellow light, appears not inconsiderably 

 increased. 



4. Blue and Violet Light. 



The blue glass absorbs almost completely the red and 

 orange to D, transmits the yellow and yellowish-green rays 

 from D to E, then again absorbs pretty completely the green 

 from E to b, and from that point completely transmits the 

 blue and violet. 



Macroscopically the ground-colour of the retina appears 

 changed to a dull " violet." The numerical proportion of red 

 to green rods is the same as after the operation of green 

 light. The latter appear of a peculiar dirty green, and their 

 presence and colour alone cause the dulness of the violet tint 

 of the retina ; for under the microscope the majority of the 

 rods appear of a completely clear bluish-red tint, which in 

 paling passes to a distinct light violet. 



It is worth notice that all these characteristic changes of 

 colour by the various kinds of light were produced in re- 

 peated ex})eriments with extraordinary constancy ; so that I 

 was soon able by examining a retina to distinguish with 

 certainty whether it had been exposed to blue, green, or red 

 (or yellow) light. It is indeed this constancy in the results 

 which induces me to publish them now, without waiting for 

 their confinnation by a secoiul series of experiments carried 

 out with truly monochromatic lights. I am on the point of 

 beginning such experiments; in addition I intend also to 

 study the effect of more intense light of different kinds, a 

 medium intensity having been only observed by me hitherto. 



