448 DR WILLIAM POLE ON THE PRESENT STATE OF 



1. What relations have the two colours seen, and their "equilibrium" (i.e., the 

 patient's white), to the colours and the white of normal vision ? 



2. What is the exact nature of the variations which are found to occur in different 

 individuals ? 



It is somewhat humiliating to have to say that, in England, these questions are often 

 answered merely by reference to assumed theories ; but we will rather, here, act on 

 Bacon's principle, so aptly quoted by our President in his Opening Address of this 

 Session, " Non fingendum, nee excogitandum, sed inveniendum quod Natura faciat." 



II. 



THE RELATIONS BETWEEN THE DICHROMIC COLOUR-SENSATIONS AND 



THOSE OF NORMAL VISION. 



This is a question which has been mooted ever since the nature of dichromatism has 

 been established, and it has given rise to much controversy. Its treatment is hot easy, 

 because colour is a subjective sensation, which does not admit of any direct and 

 independent description. Evidence must be obtained indirectly, and this is generally 

 done by comparisons made in various ways. 



One of the simplest modes is by what are called matches of colours, which are now 

 used very commonly for the detection of colour-blindness. A colour-blind person will 

 sometimes select two skeins of wool which he says "match," i.e., look alike, to him, 

 while to a normal-eyed person they appear glaringly different. This comparison furnishes 

 at once the important evidence that the person's vision is abnormal ; and by following up 

 the plan, carefully and systematically, much information may be obtained as to the 

 general nature of his vision. 



Now it happens that, although matches made by the colour-blind person may seem 

 absurdly unlike to the normal-eyed ; when the experiment is reversed we get a 

 different result ; for it is found by experience that, as a general rule, matches made by 

 the normal-eyed will also be matches to the colour-blind.* And this furnishes useful 

 evidence of a more positive kind ; for it is understood to prove that the colour-sensations 

 of the colour-blind are generally of the same nature, and derived from the same sources, 

 as those of the normal-eyed ; or, in other words, the two colours of the dichromic eye 

 must be two of the hues known in ordinary vision. 



Having this guide, we may proceed to inquire what hues they are ? and we may look 

 for two kinds of evidence on the point. 



A. We may first inquire what is the testimony of the dichromic patients as to their 

 own sensations ? How is it conveyed ? And what is its value ? 



* That is, subject to exceptions caused by personal variations, — which, according to recent researches, may 

 sometimes be considerable. 



