230 FRANZ BOLL. 



Rays of different wave-lengths act upon the retina in 

 different ways. The red colour of the retina is not changed 

 at all by the rays of the greatest wave-length (red and yellow 

 light). A marked alteration of the ground-colour is at 

 once produced by the rays from the middle of the spectrum 

 (green); and the strongest alteration^ is caused by the rays 

 of the shortest wave-length (blue and violet) from the ex- 

 treme end of the spectrum. Probably these three categories 

 of colour are identical with the three primary colours postu- 

 lated by the theory of Young and Helmholtz. 



It is not yet possible to found a theory of colour-sensation 

 on these observations, but I may already point out that a 

 great part of the observed facts are in harmony with the fol- 

 lowing theory : 



" That by the action of the various colours objective colour- 

 changes are produced in the layer of rods of the retina {i. e. 

 in a part of the nervous system), which ?ixe identical with the 

 substance of the sensations and subjective ideas to which 

 they give rise." 



If it should be possible to carry out this conception really 

 and thoroughly for the theory of colour-sensation, a com- 

 pletely new solution of the primeval question concerning the 

 reality of the substance of our sentient perception would 

 directly arise from it. 



II. 



I wish to make a few additions to my previous communi- 

 cation. 



I, The constant sunny days of the second half of January 

 have enabled me to determine with greater accuracy than 

 before the duration of the time in which the vision-red is 

 destroyed by sunlight and again restored by darkness. If a 

 dozen frogs are simultaneously taken from complete darkness 

 and exposed to the sun in glass vessels, and if a pair of eyes 

 be examined every five minutes, we find that already in the 

 first five minutes a considerable paling of the vision-red has 

 taken place ; after ten minutes a weak glimmer of the red 

 colour may still be demonstrated; but this is very rarely to 

 be met with after fifteen minutes; commonly after this time 

 the retina is already completely colourless ; finally, after 



' In the slight alteration produced by the green rays, and the much 

 stronger change effected by the blue and violet rays in the ground colour 

 of the retina, there may perhaps be found an explanation (assuming the 

 identity of these relations in man) of the fact that the majority of colour- 

 blind people cannot di&tinguish red from green, while red and blue are con- 

 fused by very few only. This latter (higher) degree of colour-blindness 

 appears, however, always to include the lower stage of red-green blindness. 



