ON COLOUR PHOTOMETRY. 
549 
Table Y. —Extinction Curves of Light transmitted through Blue and B-ed 
Glasses. 
Scale 
number. 
Wave length. 
Reduction to produce 
extinction, E. 
(Sector readings). 
Comparative 
Luminosities of 
unreduced beams, L. 
E x L. 
r 14-5 
4363 
4 
148 
592 
135 
4.336 
4 - 5 
132 
594 
12-5 
4309 
5 
120 
600 
11-5 
4284 
5-5 
108 
594 
CO 
10-5 
4258 
6-2 
96 
595 
d 
9-5 
4233 
7-0 
86 
602 
CL 
8-5 
4209 
8-0 
76 
608 
7-5 
4186 
8-8 
68 
594 
6'5 
4163 
9-8 
62 
617 
bo 
5-5 
4141 
11-0 
56 
616 
d 
o 
4-5 
4118 
12-0 
50 
600 
r—< 
rS 
3-5 
4094 
13 
45 
585 
2-5 
4070 
16 
37 
592 
1-5 
4047 
19 
35-5 
598 
•5 
4025 
25 
24 
600 
— -5 
4023 
35 
17 
595 
Tj 
r 64 
7212 
73 
8 
584 
0) 
63'5 
7147 
45 
13 
585 
— go 
T n W > 
62-5 
7022 
25 
23 
575 
61-5 
6900 
12 
49 
588 
GO-5 
6785 
5-5 
101 
555 
^ 59 - 5 
6675 
.3 
168 
504 
From Table Y. it is seen that from the extreme violet end of the spectrum, to 
No. 14*5, the luminosities of extinction are practically the same ; in fact the curve at 
this part is horizontal; the same is the case with regard to the part between scale 
No. 61 ‘5 and the extreme red end of the spectrum. 
This seems to confirm the view that the colour sensation of the eye for each of 
these parts is a simple one. These results have been, as already said, incorporated 
in Tables III. and IY. 
In the diagrams, fig. 41, two curves of extinction are given. One shows what 
proportion of the beam, at different parts of the spectrum, is just not visible to the 
central portion of the eye, the other, the curve with regard to the ivhole eye. These 
curves correspond with each other, except where the absorption by the yellow spot 
takes place. The part of the retina which appears most sensitive to the light of this 
part of the spectrum is about 20° below the centre, and about 45° from the vertical 
line. The light is certainly most persistent at this point. 
If the reciprocals of the ordinates of either of the curves just referred to—that is to 
say, of the reduction of the beam at different points—-be taken as ordinates, the 
curve so constructed may be called a “persistency” curve, and should relate to some 
colour sensation in our eyes. Such curves (with ordinates so reduced that the 
