82 



Dr. G. J. Burch. 



[Dec. 22, 



brush-marks from being seen, I devised the plan which constitutes the 

 novelty of this test. 



The card lies at the bottom of a box like that of an old-fashioned stereo- 

 scope, but larger. In front of the card, at a distance of 6 or 8 inches from it, 

 I fix a sheet of perforated zinc, and the observer looks through a short tube 

 in the top of the box, containing a convex lens of about 8 diopters, focussed 

 on the zinc. The card is so far beyond the zinc that it is necessarily out of 

 focus, even for those possessing full power of accommodation, all that can be 

 seen through the holes being merely the colours, which form a mosaic of 

 coloured dots, not unlike those of Prof. Stilling's test, from which, in fact, I 

 got the idea. 



But the method has this advantage over Stilling's cards — given the colours, 

 any design can be painted in a few minutes. It is astonishing how soon the 

 exact description of a test-card becomes known if many people from the same 

 neighbourhood are tested ; hence the importance, even in the absence of any 

 conscious attempt to deceive the examiner, of being able to vary the lettering. 



A year later, Lieut.-Col. Scott said that the colours were no longer a match, 

 and he could even read the red letters through the perforated zinc. On close 

 inspection there appeared to be a sort of whitish bloom on the surface of the 

 vermilion, but so very slight that few people would have noticed it. He 

 found that fresh vermilion still matched the green, as did the old after the 

 application of oil. From this it would appear that the green was unchanged. 

 The whitish bloom would represent to him an addition of blue, to which he 

 was exceedingly sensitive. 



II. For the next card the pigment selected was geranium-red, which differs 

 from vermilion by the addition Of blue and violet, these colours being distinct 

 to Lieut.-Col. Scott. This was found to match with a sort of French grey or 

 slate-colour with no tendency to green. On the grey half of the card I 

 painted 00 in blue and NOT in geranium. The first word he could see at a 

 glance, but the second was invisible to him, although to me it was more 

 conspicuous than the other. 



III. For the third card we took emerald green, mixed with a little yellow, 

 to cut off some of the blue rays. This was found to match with a rather rich 

 yellow ochre, made with Mars yellow, Oxford ochre, and a little burnt sienna. 

 The letters P.T.O. were painted in the yellow ochre mixture on the green, 

 and B.S.V.P. in emerald green upon the yellow ochre background. 



He could not see either, the whole card appearing one uniform green to 

 him. But he immediately detected the letters O.K. upon the green and 

 A.B.C. and Z upon the yellow ochre in a salmon-pink made by adding a very 

 little geranium-red to the yellow ochre, instead of the burnt sienna. 



