XX111 



rectly, and in fact his assumption gives an extraordinarily simple and 

 clear view and explanation of all the phenomena of the physiological 

 theory of colours" (' Phys. Opt.,' 1867, p. 291). So much was 

 Young in advance of his age that his theory was hardly noticed, 

 " bis ich selbst und Maxwell wieder auf sie aufmerksam machten " 

 ('Phys. Opt.,' 1867, p. 307). 



From this standpoint von Helmholtz did not recede ; but in his 

 later years he was impressed with the difficulties which attend the 

 explanation of colour blindness on the hypothesis that in such cases 

 one of the three fundamental sensations is weakened or absent. 



In the twentieth appendix to the first edition of the ' Physiolo- 

 gisclie Optik ' he proved that he did not even at that date (1867) con- 

 sider this to be the only possible explanation, but that the phenomena 

 of cclour blindness might be due to the fact that the forms of the 

 intensity curves of the three fundamental colours were abnormal, 

 rathar than to an incapacity to appreciate one of the fundamental 

 sensations. 



It is unnecessary to discuss here the modifications of his earlier 

 views of which there are proofs in the latest edition of the great work 

 on optics. In January, 1893, a very interesting comparison of the 

 opinions expressed in the two editions was contributed to the ' Philo- 

 sophical Magazine ' by Dr. W. Pole, F.R.S., who is himself a type 

 C£se of, and an authority on, colour blindness. It is therefore suffi- 

 cient to say that von Helmholtz gave a new determination of the 

 t-iree fundamental colours, which were found to be red, green, and 

 blue, but to be very much more saturated than any colour which 

 occurs in the spectrum. His final word on the subject of colour 

 blindness was that according to his latest " generalisation of the 

 theory of dichromic vision we get rid of the separation of the green- 

 blind and red-blind into two sharply divided classes, a separation 

 which does not appear to have been completely established by obser- 

 vation. It is also shown that the want of correspondence between 

 the absent colour of the dichromic system, and one of the funda- 

 mental colours found by us, does not involve any insoluble contra- 

 diction." 



In 1853 Helmholtz communicated to the Academy of Berlin a pre- 

 liminary note " On a hitherto unknown alteration in the Human 

 Eye during a change of Accommodation." He was at that time im- 

 perfectly acquainted with researches which had recently been made 

 on the same subject by others, and in his full paper, which was not 

 published till 1856, he frankly admitted that he had been anticipated 

 in several points. In finish and delicacy, however, his work far 

 exceeded that of his predecessors, to which, moreover, he made some 

 important additions. He invented the ophthalmometer for measur- 

 ing the changes - in the magnitudes of the images formed by reflec- 



