502 DR WILLIAM PEDDIE ON A 



beyond the lines H. I asked Mr A. if he would be as well satisfied in calling that 

 half of the spectrum green, and he said that he would be so. 



From the preceding observations, it seemed that the case was one of red-green vision 

 or of blindness to yellow and blue. But there was this striking peculiarity — that Mr A., 

 if he were asked to place the cross wires at the extreme limit of vision at either end of 

 the spectrum, invariably placed them at points corresponding to full normal vision ; 

 whereas, in all cases of yellow-blue or " violet" blindness hitherto described — so far as I 

 know — the blue end of the spectrum is considerably shortened, and sometimes the red 

 end is shortened also. 



I then showed Mr A. the two complementarily-coloured images which are produced 

 by passing plane polarised light through a plate of quartz and a double-image prism 

 and asked him if he could make the colours alike by rotating the prism. He said 

 that it was impossible for him to do so. The quartz was of such a thickness that the 

 complementary colours were, on the whole, reddish and greenish respectively. I then 

 replaced the quartz by a thinner plate, which gave bluish and yellowish complementary 

 colours, and he said that he could then get equality. The colours which he asserted 

 to be alike were a pale blue and a pale straw colour respectively. This verified the 

 conclusion that the case is one of blindness to yellow and blue. 



On the other hand, when the thicker quartz plate was used, I asked Mr A. if he could 

 make one of the images colourless ? He replied, with some hesitation, that he could ; 

 and I was somewhat surprised to find that the colour then was a strong blue-green. 

 This seemed to indicate considerable weakness in the green sensation. 



I next showed Mr A. a dark blue, strongly-coloured powder. He said that it was 

 almost black, and he remarked also that a dark blue sky seemed almost black to him. A 

 strongly-coloured yellow powder he pronounced to be almost colourless. Subsequently, 

 I showed him an intimate mixture of these two powders, which was distinctly of a green 

 colour though it was greyish, and he said that it also had no colour. 



Colour-Disc Tests. 



Colour equations were then obtained in the usual manner by means of the rotation 

 of coloured discs. These are given below. The numbers, as usual, represent the angular 

 measure of the various coloured sectors. The symbols R, Y, G, B, Bk, W, represent 

 respectively red, yellow, green, blue, black, and white. The red was a strong scarlet, 

 containing some yellow ; the green was a strong emerald green, containing some yellow ; 

 and the blue was Prussian blue, though somewhat dull in tone. The yellow might 

 be called pure. 



Limits between which the true match lay were taken. Thus, when a green disc was 

 matched with black and white, the limiting results were 



360G = 130W + 230Bki j 



360G = 110W+250Bk J 



