120 Scientific Proceedings, Royal Dublin Society. 
conditions, albeit, as Sir W. Abney has shown, the selective absorp- 
tion exercised by the yellow spot does prevent accurate work in 
matching colours when that part of the retina is used. This occurs 
when the colours to be matched are held at such a distance from 
the eye that the retinal image falls on the yellow spot. When the 
eyes are near to the colours to be matched, say, at the distance of 
about 2 feet, the retinal image is large, and extends considerably 
beyond the region of the yellow spot. When, however, the colours 
are held at a considerable distance from the eye, the retinal image 
is small, and may be made to fall entirely within the area of the 
yellow spot. Under such circumstances the blue-green rays are 
largely absorbed by the spot, and an error in matching those 
colours then occurs.! 
That there is a definite selective absorption of light by the 
macula lutea in the living eye was shown by Stokes—quoted by 
Maxwell’—who employed a weak solution of chromium chloride, 
through which a white surface was viewed. The solution is of a blue- 
green colour, transmitting the blue, green, and also the red rays, but 
opaque to yellow. Accordingly, when a bright cloud or a sheet of 
white paper, illuminated by the sun or by a beam from the electric 
light, is viewed through this solution, a dark reddish patch will be 
seen in the midst of the bluish-green ground, owing to the 
yellow spot being largely opaque to the blue-green rays of the 
incident light. As already mentioned, the solution suggested by 
Mr. Whitmell is even more effective—the double oxalate of chro- 
mium and potassium, which is quite opaque to the yellow and 
orange rays. A bluish gelatine film also serves very well, and 
is convenient to use. The yellow spot can also be seen if the 
eye traverses the spectrum. 9 
Maxwell states that in persons of a dark complexion, the 
yellow spot is more pronounced than in fair persons. I do. 
not know if examination of extirpated eyes has confirmed this: 
it would be interesting to know. So far as my own experience 
1 See Colour Measurement and Mixture, by Sir W. Abney, D.c.u., &c., p. 117. 
* British Association Reports, 1870, page 40; also Maxwell’s Collected Papers,. 
vol. ii., page 279. 
3 According to Tscherning (Physiological Optics, p. 198), there is no trace of a 
yellow colour seen in the macula when the living eye is examined by the ophthalmo- 
scope! Is this really the case, or is it due to the absence of the yellow spot in 
certain individuals ? 
