The Scattering of Light. 
265 
1912 - 13 .] 
been made incidentally in connection with researches directed to quite 
other ends. Thus, for example, in photographic research, when measure- 
ments are to he made of the densities of negatives, it is necessary to 
have regard to the tact that such negatives not only absorb light but 
also scatter it ; and this has led to experiments on the scattering of light 
by negatives, and to the consideration of certain related matters.* 
And again, the introduction of various modern illuminants in which a 
great intensity of light proceeds from a source of small dimensions has 
created a demand for diffusing screens,]- and a certain amount of data 
is available in regard to these. The scattering, however, is often obtained 
either by irregular reflection, j or by particles embedded in the glass which 
are so small as to be of dimensions comparable with the wave-length of 
light itself, § and neither of these cases, of course, has any connection with 
Christiansen’s experiment. 
In view then of the scantiness of the existing information on the 
subject, and of the desirability of adding to it, I began a series of experi- 
ments ; and the purpose of this paper is to give some account of the results 
that have been obtained up to the present time. 
§ 2. Apparatus. 
On reference to fig. 1 it will be seen that the light from the ground- 
glass plate G x under examination is reflected at the mirror M, and con- 
densed by the lens L 2 so as to pass into the slit of the spectrophotometer S 
by which its intensity is measured. This measurement is effected by 
balancing its light against that from another piece of ground glass G 2 
illuminated by the movable Nemst lamp N 2 , which can be pulled back- 
wards and forwards at will along the rails R. The light from G 2 passes 
above the mirror M and is likewise condensed by the lens L 2 into the slit 
of the spectrophotometer. The latter is an instrument on the flicker 
* E.g. Sir W. de W. Abney, Jour. Soc. Chem. Ind ., 31st July 1890, p. 772, “ On the 
Accuracy of the Grease-Spot Photometer for Measuring the Density of Photographic Plates, 
and a Note on the Sector Photometer.” “ A Demonstration of Scatter in Turbid Liquids,” 
W. Scheffer, Brit. Jour. Phot., p. 941, 9th Dec. 1910. C. Jones, Photographic Journal , 1898- 
1899, and Andre Callier, p. 200, 1909. Theory of the Photographic Process , Sheppard and 
Mees, pp. 38 and 114. See also The Brit. Jour, of Photography for 30th Aug. and 13th Sept. 
1912, in connection with the scatter error in wedge photometry. 
t See, e.g., Electrical Photometry and Illumination, H. Bohle, p. 153 et seq. 
f On the subject of diffuse reflection, see, e.g., Trotter, Illumination , its Distribution and 
Measurement, p. 93 et seq. Outlines of Applied Optics, P, G. Nutting, p. 165 et seq. F. H. 
Gilpin, Trans. III. Eng. Soc., v. 854-874, Dec. 1910. PI. R. Wright, Phil. Mag. (5), No. 49, 
p. 119, 1900. 
§ See, e.g., Physical Optics, Wood, chapter on “ Scattering by Small Particles.” 
