KNOWLEDGE AND OPINION IN REGARD TO COLOUR-BLINDNESS. 477 



complicated his excellent wool-testing arrangements by introducing devices for dis- 

 covering whether the patient is " red-blind " or " green-blind," — a distinction which he 

 himself now candidly repudiates. 



I therefore agree with the opinion of the Ophthalmological Society of Great Britain 

 (see " Data," letter P) that, for practical purposes, the testing should be limited to simply 

 ascertaining the existence of the defect ; — if any further special investigation of its nature 

 is desired, it should be undertaken by scientific experts, who will know, or at least ought 

 to know, the proper mode of conducting the inquiry. 



On the Law of Heredity of Colour-Blindness, 



An authoritative statement has lately been published in England, that colour- 

 blindness descends from father to son; but this is at variance with the recorded facts 

 and opinions. The true law of heredity, as settled chiefly by continental investigations, 

 is, with occasional exceptions, that the affection is transmitted by a male sufferer not to 

 or through his sons, but to his grandsons through a daughter, who, however, is free from 

 it herself; so skipping over one generation. Donders mentions this as " the unanimous 

 testimony of experts." 



In my own case I can testify that neither my father nor my mother had any such 

 defect, nor have any of my children, or my sons' children. In a case examined by me 

 (see Phil. Mag., July 1892, p. 102), similar partial corroboration is given. 



But I am able to add a full and striking confirmation of the law from English experi- 

 ence. Some years ago I was favoured by a voluntary communication from a gentleman, 

 who, although he knew nothing of the law in question, had nevertheless deduced it 

 quite independently from the records of five generations of his family. And as this is a 

 most circumstantial and interesting statement, I have obtained the permission of the 

 writer to publish his letter in the " Data," AF. There are objections to giving the 

 names, but the family stand high in reputation ; and my informant has evidently taken 

 much pains in the collection of the facts. 



It will be seen that they fully bear out the established rule. 



Early Indications of the Tetrachromic Theory. 



Although Professor Hering, as stated on p. 466, has undoubtedly the merit of 

 originating the present interest in the tetrachromic explanation of colour-blindness, it is 

 worth mentioning that the idea of four fundamental colours, as contrasting with the 

 trichromic combination, adopted by Young, had existed previously. 



The four colours, red, green, blue, and yellow, were selected, with white and black, 

 as " simple colours " by Leonardo da Yinci. In his Trattato di Pittura, written about 

 a.d. 1500, he not only carefully describes how objects should be coloured, but he gives 



