ON COLOUR PHOTOMETRY. 
447 
moment of extinction, and thence to deduce the luminosity of the coloured light. 
This method was used subsequently by Dr. Draper. 
Observations on Solar Spectrum. 
6th July, 1885. 
17th November, 1885. 
Scale reading. 
Intensity. 
Scale reading. 
Intensity. 
42-2 
0 
42-54 
0 
43 
6 
43-3 
4-7 
43 6 
16 
44-3 
27 
44-6 
48 
44-55 
47 
45-5 
80 
45-17 
64 
46-8 
100 
45-80 
80 
47-7 
90 
46-55 
97-6 
487 
78 
47 
98-8 
49-7 
48 
47-65 
92 
50-5 
32 
48-3 
80 
51-5 
22 
48-92 
64 
53-5 
11 
49-55 
44-5 
55-4 
6 
49-67 
42-5 
57 
4 
50-16 
30 
50-8 
19 
52-05 
12-5 
52-2 
12 
53-3 
8 
54-5 
5-5 
55-8 
4 
Fig. 7, Plate 25, also gives Vierordt’s results on a bright summer’s day; for the 
sake of comparison we have reduced his maximum to agree with ours. 
His table is as follows :— 
Position in Spectrum. 
Light Intensity. 
a — a\ B .... 
80 
B|C - C . 
493 
C - C/ t D .... 
1100 
C^r - Ci*D .... 
2773 
CyfU - D. 
6985 
D - D T yE .... 
7891 
DTs® - E. 
3033 
b — F . 
1100 
P - F^G . 
493 
FfG - FfG . 
90 
G - GffH .... 
35-9 
GMH - GffH .... 
131 
GfiH - H . 
5-8 
Beyond H . 
•9 
Now in Vierordt’s estimation of the intensity of another source of light he has taken 
the mean of the two positions in the spectrum, and indicated by straight lines joining 
these points the light intensity of the different parts. We show by a chain dotted 
line the curve formed in this way. The lines joining the less luminous ends of 
