115 
We will, however, first give a short description of the interfero- 
meter with the aid of figures 1 and 2). 
2. Fig. 1 gives a schematic representation of the instrument. The 
rays furnished by a source of light S, made parallel by lens L, 
pass through the identical vessels C, and C, filled with the same 
liquid, and then through a screen provided with two slits R, and R,, 
are reflected as two separate beams by mirror M, and after having 
been united again by lens L, they form an interference image in 
O. When white light is used this image consists of a central bright 
band bounded by two dark ones; the bright bands following on 
them on either side have coloured edges. When now C, is filled 
with a solution which has a greater index of refraction than the pure 
solvent in C,, this interference image is displaced in consequence of 
the lengthening of the optical path, e.g. to O’. It can be brought 
back to the zero position O by turning the compensator plate 
P, (P, is immovable), through which the optical path is again 
artificially shortened. The angle, over which P, has been rotated, 
is a measure for the difference of index, hence indirectly for the 
concentration of the solution in C,. 
4 
4 
4 
4 
Fig. 2. 
Fig. 2 represents a horizontal and a vertical cross-section of the 
interferometer more in details. The white light of an Osram lamp 
F is concentrated by lens A and prism K, on the very narrow slit 
5, which forms the secondary source of light. The passage of the 
') A fuller description is found in the following places: 
F. Löwe, Zeitschr. für Instrumentenkunde 1910, 321; F. Lowe, Physikalische 
Zeitschr. 11, 1047 (1910). 
P. Hirscu, Fermentforschung 1, 33 (1914). 
L. H. Apams, Journ. Amer. Chem. Soc. 37, 1181 (1915). 
8* 
