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THE THRESHOLD OF VISION FOR DIFFERENT COLOURED LIGHTS. 115 
In the case of fovea! vision for observers of class I. we have to do with cone vision 
and hence the luminosity curve to be employed is that corresponding to a bright 
spectrum. In Table V. are given the values of the luminosity obtained by W. with a 
bright spectrum and with the Nernst light as a source. 
Table V. — Luminosity Curve, Bright Spectrum (Cone Vision) for W. Source of 
Light, Nernst Glower. 
SSN. 
Luminosity. 
SSN. 
Luminosity. 
62 
3-0 
44 
75-0 
60 
11-5 
42 
59-5 
58 
27-0 
40 
42-5 
56 
56-0 
38 
27-5 
54 
83-0 
36 
15-2 
52 
94-5 
34 
8-2 
50 
99-5 
32 
4-7 
48 
99-0 
30 
4-5 
46 
90-0 
28 
4-0 
If we multiply the extinctions (that is the reductions of the various parts of the 
spectrum to give the threshold) by the corresponding luminosities, the products will 
give us numbers which are proportional to the amounts by which the different colours 
of the spectrum must be reduced to give the threshold. This supposes each colour 
before reduction to be equally brightV 
In fig. 17 are given the foveal curves for the observers of class I., the ordinates 
being the product of extinction into cone luminosity. The curves are rather 
irregular, but it must be remembered that in the case of observers of this class the 
foveal observations are very much more difficult to obtain than extrafoveal values. 
If we take the mean of all the observations the product of extinction into luminosit)^ 
is very nearly constant except in the neighbourhood of SSN 40 (527^/x), where there 
is a decided rise. It is, however, to be remarked that this point in the spectrum 
appears to be the most difficult to obtain good central fixation, and W., who has 
had much more experience in making these observations than any of the others, 
does not obtain a high value at this point. On the whole we think we may 
conclude that for class I. at the fovea the threshold values throughout have the 
same fraction of the original brightness ; this original brightness being the same for 
all colours. 
* With class I. before reduction each colour produces an equal sensation by means of cones only with 
foveal fixation or with the rods also for foveal vision with class II., and for extra-foveal vision for both 
classes. 
YOL. CGXYI.-—A. 
R, 
