194 
Two Cases of Trichromic Vision.* 
By F. W. EpRIDGE-GREEN, M.D., F.R.C.S. 
(Communicated by Dr. F. W. Mott, F.R.S. Received January 24,—Read 
February 23, 1905.) 
1. One case (Professor J. J. Thomson) sees only three colours in the bright 
spectrum—red, green, and violet. He can distinguish nothing of the nature 
of pure yellow, like the sensation given him by the sodium flame in the 
spectrum. There is no definite colour to him at the portion of the spectrum 
where the normal sighted see pure blue. Reddish-green would describe the 
orange and yellow regions and greenish-violet the blue. 2» 5950 (orange- 
yellow) is the point which differs most from red and green. There was no 
shortening of either end of the spectrum. The point of junction of the red 
and green differed somewhat in repeating the observations because of his 
great sensitiveness to simultaneous contrast. It was, however, always in the 
orange or orange-yellow, never in the yellow of the normal sighted. 
Difference of Hue Perception.—I then tested him with my apparatus for 
ascertaining the size of different parts of the spectrum which appear 
monochromatic, and found that he was defective in distinguishing differences 
of hue. A portion of the spectrum corresponding to the D lines, and 
isolated by two shutters in the eye-piece of the spectroscope, was first shown. 
The shutter on the red side was gradually opened until a difference of hue 
was seen. The monochromatic patch extended from 2» 5889 to » 6052, 
beiug exactly half as large again as that of the normal sighted, which 
occupies the space from 2» 5889 to > 5998. The monochromatic patch he 
called greenish-yellow. His monochromatic patch in the centre of the green 
bore exactly the same proportion to mine as in the case of the orange-yellow, 
being just half as large again. 
Colour Mixtures—He was then tested with Rayleigh’s apparatus for 
matching spectral yellow by a mixture of red and green. 0 of the scale 
corresponded to pure red, 25 or 90° to pure green. He made the following 
10 matches :— 
* This research was made with the aid of an instrument purchased with a grant from 
the Government Grant fund. 
