360 
PHYSICS: C. BARUS 
i8Fry, /. Amer. Chem., Soc, Easton, Pa., 34, 1912, (1268), Zs. physik. Chem., Leipzig, 
76, 1911, (385, 398, 591). 
19 Jones, L. W., /. Amer. Chem., Easton, Pa., 36, 1914, (1268). 
20 Stieglitz, Ibid., 36, 1914, (272); 38, 1916, (2046). 
" Bray and Branch, Ibid., 35, 1915, 1913, (1440). 
^ Lewis, G. N., Ibid., 35, 1915, (1448). 
TRANSVERSE DISPLACEMENT INTERFEROMETRY 
By Carl Barus 
DEPARTMENT OF PHYSICS. BROWN UNIVERSITY 
Communicated March 26, 1917 
1. Vertical Displacement of Ellipses. — In the diagram figure 1, M 
and N are half silvered, and opaque mirrors; C and C the U 
tube referred to in the preceding note and here to be removed. L is 
the collimated beam of white Hght and G T the direct vision spectroscope. 
If the fringes are too small, when horizontally centered by the micro- 
meter, the center of ellipses may be brought into the middle of the field 
of the telescope by sHding one component beam vertically over the 
other without appreciably changing the direction of the rays. In other 
words one illuminated spot at d, figure 1, is to move vertically relative 
to the other by a small amount. This may be done by placing a thick 
plate glass compensator such as is seen in figure 2 in each of the com- 
ponent beams abd and acd and suitably rotating one plate relative to 
the other, each on a horizontal axis. Very little rotation is required. 
In the same way elliptical fringes may be changed to nearly linear hori- 
zontal fringes when desirable. If the fringes are to be sharp the slit 
must be very fine. When sunlight is used with a slit not too fine, each 
of the coincident sodium lines (DiD2) frequently shows a sharply defined 
helical or ropelike structure. This is a special phenomenon, which will 
be given further consideration presently. 
The first result is interesting inasmuch as it is thus possible to dis- 
place the centers of ellipses not only horizontally as usual relative to 
the fixed sodium lines in the spectrum, but also vertically relatively to 
