550 
CAPTAIN W. DE W. ABNEY AND MAJOR-GENERAL E. R. FESTING 
maximum is 100) were constructed for the centre of the eye, and for the whole eye 
(fig. 33, C, D). Column VI. in the tables gives the ordinates. 
§ XLIX .—Extinction of Light to the Colour Blind. 
In a paper in the £ Proc. Pioy. Soc.’ (1891) one of us has given a curve of luminosity 
of the spectrum as seen by two brothers whose sensation was monochromatic. On 
comparing this with our persistency curve for the centre of the eye (which must repre¬ 
sent the luminosity of the spectrum to some sensation which we have), we were 
surprised to find that the curves corresponded except in the yellow spot absorption 
portion, when ours fell below theirs, see fig. 33. It, therefore, appears that the sensa¬ 
tion of the two brothers very nearly corresponds with what must be the dominant 
sensation in our eyes. 
Before commenting on this, it will be of interest to give a further confirmation 
of the existence of this one sensation. A gentleman, whom we will call M., had 
his vision tested. His is a case we have never tested before, and is most 
remarkable. The only two colours he saw are what he called red and black. He 
called all green and blue black, green however he called bright black, blue being 
described as a darker black. Yellow he called white. At 52 on the scale he saw a 
“little red,” at 50 “no colour”; his neutral point—if it may be so called—or the 
point where he saw the spectrum colourless, would be about 495 or about 5800. 
His luminosity curve is given at M, fig. 33. The following is the table from which 
it was plotted. The mean readings being multiplied by l - 8, the curve of luminosity 
of the red part of the spectrum almost exactly coincides with that of the authors. 
Column I. gives the scale number, II. the wave-length, III. the actual mean reading, 
IV. the last X 1‘8, V. the ordinates of the normal luminosity curve of the central 
part of the eye, VI. the difference between M.’s curve and the normal luminosity 
curve, whilst VII. gives the difference multiplied by 5‘15 to bring the maximum to 
100 for comparison with other curves. It will be seen that this curve (F., fig. 33) very 
nearly coincides with our persistency curve, except in the part of the spectrum affected 
by the yellow spot. 
No measures were taken to ascertain if the eye of M. had any central absorption, 
and, therefore, we do not know what correction should be made to the curve to make 
it comparable with the others; but taken as it is, it is remarkable how closely this, 
which represents the deficiency in M.’s sensations, corresponds with the one sensation 
of either of the brothers. In fact, it seems as if the eyes of M. and P. together 
would make up a pair of normal eyes. 
