216 Transactions of the 



The phenomenon which I have attempted to descrihe is not 

 explained by the ordinary theory of stereoscopic vision, which, as 

 given by an eminent writer, is, "that if the two images of the 

 right and left aspects of a solid body be made to fall on the retinae 

 of the two eyes in such a way as to coalesce into a common image, 

 they are judged by the mind to proceed from the single body, which 

 alone, under ordinary circumstances, is competent to produce them." 



In the case of two complementary tints viewed simultaneously 

 through the tubes of the binocular, there is certainly a coalition of 

 the fields into a common image ; but it is clear that as no colour 

 has been visible, the mind can have formed no judgment which 

 can have conduced to such a result as we have seen is thus produced. 

 May it not arise from the impression conveyed by the two optic 

 nerves reaching a common nervous centre, so that the impressions 

 from the two eyes are blended ? 



If over the upper half of the selenite above described a piece of 

 mica of exactly the same thickness as that interposed between the 

 prism and analyzing plate be cemented, but with its tension in an 

 opposite direction, the colour of the light transmitted through the 

 covered portion will be lowered from the green of the third to 

 the red of the second wave, and as the upper and lower pieces of 

 mica will (as far as they are superimposed) neutralize each other, 

 the colours in the two tubes will be reversed; but with this 

 difference, that whereas in the first instance we had the green and 

 red of the third wave, we have now green of the third and red of 

 the second, which are as before, very nearly complementary. 



On examining the slide thus prepared, we shall pass, on moving 

 it towards the observer, from fields entirely green and red, to 

 others which are bisected by these colours and their complementary 

 tints, rendering them respectively half green and red, and half red 

 and green ; these, again, will change on passing the slide onwards 

 until the field which was originally green has become red, and that 

 which was originally red has become green, but as far as colour is 

 concerned, the brain will stiU remain totally insensible to the 

 changes which have taken place. 



1 just now mentioned that if, under the circumstances then 

 detailed, one eye be closed, the colour returns faintly ; but it is a 

 noteworthy fact that on doing so with party-coloured fields the 

 colours are instantly reproduced, and, as it appears to me, with all 

 their original intensity. 



Numerous other experiments might be mentioned in which 

 objects of divers colours appear as merely varying shades of black 

 and white or grey when viewed in a similar manner ; but to detail 

 these w^ould be practically a repetition of those already mentioned. 

 Enough has, I think, been said to show how this phenomenon of 

 " colour-blindness," if I may use the expression, may be produced. 



