An investigation of the colour vision of 527 students by the Rayleigh test

Lord Rayleigh discovered, in 1881, that if homogeneous yellow is matched' with a mixture of homogeneous red and homogeneous green, some persons require much more red, others much more green, in the mixture than the normal. The former have been called red anomalies, the latter green anomalies, and both together have been grouped under the name of anomalous trichromats, because, according to Rayleigh, they are generally not diehromats. Rayleigh obtained matches for 23 male observers. Of these 16 agreed with himself within the errors of observation, and were regarded as normal. Five of the remainder were green anomalies and two red anomalies. Among seven female observers there was not one whose colour vision differed sensibly from his own. Rayleigh did not publish his data, but states that one of the green anomalies had the ratio green to red 2.09 times as great as himself, and one of the red anomalies had the ratio red to green 2.6 times as great as himself. He states that, although the number of observers is insufficient for statistical purposes, it is evident that the peculiarity is by no means rare, at least, among men; he also states that it would seem as if normal colour vision were not of the nature of an average from which small deviations are more probable than larger ones, but that this would require confirmation. In 1890, Schuster published an account of a more extensive investigation carried out with the same apparatus; 75 individuals were examined; of these three proved colour blind; four, of whom three belonged to the same family, were green anomalies, while one was a red anomaly. Schuster states: (i) that there cannot be any doubt of the real existence of small differences following the ordinary law for deviation from a mean; and (ii) that the larger differences seem certainly more frequent than the distribution of small differences would lead us to expect.