146 Mr. A. Schuster. Experiments with [June 5, 



errors, we should only have one observer in 50,000 who would read 

 twenty-one units different from the rest. While counting the different 

 members of the same family only as one, I have found three such 

 large differences among seventy-five, and Lord Rayleigh found the 

 same number among thirty observers. He also examined seven female 

 observers, none showing any decided difference from the mean. In 

 the above table numbers 9, 23, 65, and 74 are women, and their read- 

 ings 4, 1'5, 4, 3, are very consistent with each other ; on the 

 other hand, it must be noted, as a remarkable exception, that 72 is 

 also a woman ; her husband has normal sight, but amongst three 

 sons two, viz., 5 and 17, show the same peculiarity. 



It is instructive to compare the readings of numbers 28 and 50 with 

 the ratios of intensities given in Table I. It will be seen that, while 

 28 requires about 2 - 8 times as much red as green to make yellow, 

 50 requires nearly five times as much green as red to produce yellow, 

 That the ratio of red to green required by one observer is thirteen 

 times as great as that required by the other. How different will 

 compound colours look to these observers ! It seems remarkable, 

 however, that both agree in the particular wave-length which they 

 call yellow, and the actual sensation of pure colours seems therefore 

 to be the same for both. It seems difficult to explain this fact in any 

 other way than that suggested by Professor Maxwell to account for 

 some peculiarities of his own eyesight, namely, by a selective absorp- 

 tion in the yellow spot of the eye, which differs in different indi- 

 viduals. To judge from the diagram given in this paper, Maxwell 

 had, as compared with his wife, the same peculiarity of eyesight as 

 the different observers mentioned by Lord Rayleigh and number 

 5 above; this could be explained by greater absorption of green 

 in the yellow spot. But, further, Maxwell's eyes presented an 

 opposite peculiarity for the rays between the green and violet, he 

 wanting less green to produce blue than Mrs. Maxwell. This 

 Maxwell tries to explain by more pronounced absorption of the blue 

 rays than of the green. I cannot quite follow him in this explanation, 

 because the greater absorption of blue does not seem to me to affect 

 its position on the colour diagram, but only the intensity of the 

 mixture produced by green and violet. I can only account for this 

 second peculiarity of Maxwell's, that in his case the absorption of the 

 violet primary colour was stronger than that of the green. If we 

 adopt the hypothesis that the different position of the pure colours in 

 the colour diagram of different observers is due to an abscrption of 

 light in the media of the eye before it reaches the retina, we are at 

 liberty to assume that the sensation of yellow in all eyes is due to 

 an excitation of nerves sensitive to green and red respectively in a 

 fixed proportion. 



Direct experiments to determine the absorption in the yellow spot 



