PHYSICS: C. BARUS 
665 
by Cayeux have the optical properties of microcline. Termier found authigenic albite crys- 
tals in the Flysch limestone of Briangonnais. Conupare Rosenbusch, H., Elemente ^der 
Gesteinslehre, 3te Aufl.., Stuttgart, 1910, (520). "Authigenic" means "formed in situ.^' 
8 Grandjean, R, Paris, C.-R., Acad. ScL 148, 1908, (723). 
9 Daly, R. A., Geol. Surv. Can., Memoir No. 38, 1912, (50). 
1" Doelter, C, Handhuch der Mineralcheinie, Dresden, Bd. 2, 2te Halfte, 1915, (556). 
11 Chrus'tschoff, K., C.-R. Acad. Sci., 104, 1887, (602). 
12 Friedel, C, and Sarasin, E., Ihid., 92, 1881, (1374). 
13 Cf. Zschimmer, E., in C. Doelter's Handhuch der Miner alchemic, 1, 1912, (910). 
THE INTERFEROMETRY OF SMALL ANGLES. ETC. METHODS BY 
DIRECT AND REVERSED SUPERPOSED SPECTRA 
By Carl Barus 
DEPARTMENT OF PHYSICS. BROWN UNIVERSITY* 
Communicated September 21, 1917 
.1. Introductory. — It occurred to me that a number of the methods 
treated in my papers on direct and reversed spectrum interferometry 
might be used directly for the measurement of small angles and possibly 
of the distance of the source of light. Such a procedure would have an 
apparent advantage, at least theoretically, of not calling for the prelimi- 
nary superposition of the images of distant objects, as the superposition 
is inherent in the method itself. But there are large constants involved 
which make the result very problematical, unless these constants can 
be reiioved by a compensator. It is very questionable, moreover, 
whether, appreciable interferences can occur, and another difficulty 
which hampers the method is the decrease in the size of objects as 
their distances increase. A progressive investigation with the object of 
ascertaining to what degree the experiment is feasible is nevertheless 
worth while. It will be convenient therefore first to develop the 
methods without reference to the ulterior conditions which limit the 
interferences and this method has been pursued. 
2. Method with Prism. — Figure 1 is a sketch of one of the methods in 
which S is the distant source of light, from which rays d and d' strike 
the mirrors m and n, are thence reflected to the silvered sides of the 
right angled prism P. After leaving it the rays enter the spectro-tele- 
scope at T in parallel. If the proper angles are selected the prism P 
may be replaced by one of any angle or by a reflecting grating. 
Suppose now the system mPn is securely attached to a rigid metallic 
beam or rail capable of rotating around a vertical axis at its center (P). 
This is indicated in figure 2 where the direction of rays and the normals 
* Advance note from a Report to the Carnegie Institution of Washington, D. C. 
