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CAPTAIN W. DE W. ABNEY AND MAJOR-GENERAL E. R, FESTING 
iii the normal eye seems to be fairly probable. A glance at the extinction curves of the 
spectrum shows how similar in many respects P.’s is to those of H. Pt. and V. H., as 
well as to that of the normal eye. It is also worth remarking that if a bright red 
and blue be mixed together by rays coming through two slits placed in the spectrum, 
so as to form a reddish purple, the red sensation is extinguished some time before the 
blue pales to any great degree, and in that part of the spectrum where the existence 
of this monochromatic sensation is evident, the last colour visible is always bluish 
even when very faint, whilst in the yellow part of the spectrum there seems a 
tendency before extinction for a greenish hue to appear, whereas, almost to the 
moment of extinction in the extreme red, the colour is of a ruddy grey. The 
strongest evidence, however, is to be found in the persistency curves of the red and 
green blind, which only slightly differ from one another and from that of the normal 
eye, and to an extent which might be expected from the nature of the observations. 
The persistency curve (fig. 43) of V. H. differs but little in any respect from the 
normal, and this tends to show that the persistency is far greater in the blue sensation 
than in the green, in other words, that the green part of the spectrum excites the 
blue sensation in the normal eye after the light has been so much reduced that the 
green sensation has ceased to be excited. V. H. is the first case in which the total 
absence of a green sensation is an established fact, and the probable luminosity of 
such green sensation is derived by subtracting the ordinates of his curve from those of 
the curve of the normal eye. That H. R. is not totally red blind, we have on several 
occasions had the opportunity of proving. He has a slight perception of red, and 
hence the difference between his curve and that of the normal eye cannot be treated 
in the same manner, as it would not represent the luminosity of the red sensation in 
its entirety. Tables VI., VII., VIII. give the observations made by P., V. H., and 
H. R. respectively, and figs. 43, 44, 45 give their luminosity and extinction curves, 
together with the normal luminosity curve for the centre of the eye. 
The persistency of the blue sensation, or, we might say, perhaps, of the sensation 
which is confined principally to the most refrangible part of the visible spectrum, is 
very remarkable, and affords some clue to the reason of the disappearance of the red 
and green before the blue in cases of colour-blindness induced by disease. 
We believe it probable that, adopting the Young-Helmholtz theory, the three 
colour sensations obtained from these observations by colour-blind people can be 
made to form the luminosity curve of the normal eye, and, at the same time, to be in 
accordance with the colour equations which have been found by Clerk Maxwell, as 
well as by ourselves. 
M.’s observations of extinction were sometimes erratic, and we therefore cannot 
make much use of the results. But they appear to afford proof that his dominant 
sensation is not more than yso as powerful as that of the normal eye, and may even 
be considerably less. An inspection of his results also shows that the extinction of 
the part of the spectrum in the red very closely resembles that of the normal eye in 
