PROFESSOR AT KONIGSBERG 133 



alters more rapidly with intensity of illumination than any other 

 portion of the spectrum, and Helmholtz was able to detect a 

 whole series of distinct tones of purple. He could not then 

 extend his investigations of the sensibility of the retina to the 

 ultra-violet rays, since the glass prisms he was using did not 

 show sufficient of the ultra-violet spectrum. But he raised 

 two other very interesting questions as to the relations of the 

 wave-lengths of complementary colours, and the relations of 

 intensity required if the mixture of simple complementary 

 colours is to produce white. He succeeded in answering these 

 questions quantitatively, and concluded on the ground of his 

 measurements of the brightness of the colours whose mixture 

 produces white, that there must be differences of saturation 

 in the various simple colours, violet being the most, and yellow 

 the least saturated. The treatise, which has been fundamental 

 for all later work of the same kind, concludes with an inquiry 

 into the validity of Newton's Colour Circle, which Helmholtz 

 designates as one of the most brilliant inspirations of that great 

 thinker. 



After a long delay he received two rock-crystal prisms from 

 Oertling in Berlin, which he had ordered through du Bois 

 for his earlier experiments, and from which he obtained an 

 ultra-violet spectrum, more than twice as long as that given 

 by the glass prisms. In the paper which he sent immediately 

 to Poggendorff, 'On the Sensibility of the Human Retina to 

 the most Refrangible Rays of Solar Light/ he propounds the 

 important, but highly complicated problem, whether the retina 

 sees the ultra-violet rays directly, like the other colours of 

 the spectrum, or fluoresces under their influence, and whether 

 the blue colour of the ultra-violet rays is light of lower 

 refrangibility, which is first developed in the retina under 

 the influence of the violet rays. By varying the methods 

 hitherto employed, he showed that the human retina is capable 

 of directly perceiving all the rays of the sun's light, the re- 

 frangibility of which exceeds that of the ultra-red rays ; while 

 further under the action of the ultra-violet rays the retinal 

 substance scatters mixed light of lower refrangibility, the total 

 colour of which is not pure white ; and lastly, the fluorescence 

 of the retina is inadequate to explain the perception of the 

 ultra-violet rays in general. He found that the tolerably 



