ON COLOUR PHOTOMETRY. 
449 
“agreed very well.” If a confirmation of the theoretical loss which light undergoes 
by passing through a turbid medium be obtained by experiment, it is evident that 
the law of the effect of small particles in scattering light is also confirmed, since the 
former is deduced from the latter. 
According to Lord Rayleigh X=X 0 € _/hA ~‘, where I 0 is the intensity of the incident 
light, x the thickness of the turbid medium, and k a constant independent of the wave¬ 
length which is denoted by X. X then represents the intensity of the light which 
passes through. 
To test the matter experimentally, a glass trough was filled with water which was 
apparently quite clear, and in which the beam from the electric lamp discovered but 
some few floating motes. This trough was placed between the slit of the spectroscope 
and the lens used to focus the image of the crater on it, the length of water traversed 
by the beam being 6 inches (exclusive of the glass walls). The curve of luminosity 
of the spectrum was then obtained. A solution of mastic varnish in alcohol was 
prepared, various quantities of which were dropped into the water in the trough for 
different experiments, the extremely fine precipitate so formed remained in suspension 
without apparent change for days, the trough remaining in its place throughout. 
“Curves of luminosity” of the beam through the clear and the turbid water were 
obtained and plotted on a large scale, and the relative intensities of the beam through 
the clear and turbid water thus determined. In order to obtain the best results it was 
found advisable to slightly increase the light admitted to form the spectrum when the 
water was turbid, on account of the difficulty of measuring a feeble light with 
accuracy. 
The following are the measurements in detail of one pair of curves 
Scale-reading 
of spectrum. 
Intensity of colour 
Through the 
clear water. 
Through the 
turbid water. 
24 
•85 
2-3 
25 
1-4 
2-7 
27 
2-3 
4-0 
30 
4-4 
6-5 
6448 
31 
5-0 
8-0 
6374 
35 
io-o 
15-0 
36 
11-2 
17-0 
6210 
39 
17-0 
24-0 
42 
22-5 
29-7 
6061 
44 
25 - 5 
31-0 
47 
27-3 
32-0 
5900 
48-5 
28'5 
31-5 
52 
29-2 
29-2 
55 
28-5 
27-0 
5589 
59 
27 
23-5 
5459 
64-5 
24-3 
17-5 
4602 
66-5 
22-5 
15-6 
3 "M 
MDCCCLXXXVI. 
