442 DR WILLIAM POLE ON THE PRESENT STATE OF 



I propose — I. To give a brief explanation of the scope of the inquiry. 

 II. and III. To investigate at some length two questions, in regard to which there 

 has been much controversy and there is still some difference of opinion. 



IV. To collect some further facts, statements, and opinions of a more general kind 

 from authoritative sources. And 



V. To attempt to draw some useful inferences from the whole. 



It will frequently be necessary, in order to prove or explain the statements made, to call 

 special attention to writings quoted as authoritative ; and as many of these, particularly 

 when of foreign origin, are difficult of access, I have arranged the necessary extracts, 

 for facility of reference, in a collection of " Data," which, with a bibliography of works 

 referred to, will be found in the Proceedings of this Society for 1893, vol. xx. page 103. 

 In the present paper these extracts will be quoted as " Data," with a distinguishing letter. 



SCOPE OF THE INQUIRY. 



It is desirable to state at the outset that the present inquiry does not extend to 

 colour-blindness generally. This term is used to include several kinds of defective vision 

 of colours. For example, patients have been found who, although they appreciate light 

 and shade, have no perception of colour proper, i.e., as distinguished from light generally ; 

 this defect may be called Achromia Vision* but it is very rare. Then there are persons 

 who see tivo colours ; this defect is called Dichromic Vision, and is the most common of 

 all. But even in this there are some rare varieties, in which, if they were well sub- 

 stantiated, the colours would be so irregular as to demand special classification. We need 

 not consider them now ; it must suffice to say that, according to general experience, by 

 far the great majority of cases of dichromic vision (and, therefore, the great majority of 

 cases of colour-blindness) are of a peculiar kind, which is characterised by great mistakes 

 in regard to the colours red and green, often confounding them with one another. This 

 is called Red-green Blindness, and it is the only kind that will be treated of in this 

 paper. 



My own vision is allowed on all hands to be a perfect typical example of this defect, 

 and as Professor Rutherford has paid me the compliment of mentioning my connexion 

 with the subject, I may be pardoned for saying a few words on its early history. 



* I take this term from Donders ; the defect has sometimes been called " Monochromic " vision, hut this is inap- 

 plicable if we assume that colour is something distinct from the sensation of ordinary light, as we do when we use the 

 now well-established term " Dichromic." On this view it is difficult to understand how there can be any monochromic 

 vision, as, if the light is at all varied by refraction, it must be broken up into at least two colours. No doubt white 

 has often to be treated as a colour-sensation ; but the popular use of the term " coloured," as distinguished from the 

 simple sensation of light, justifies the above nomenclature. 



