338 
PHYSICS: C. BARUS 
eccentric. This however is not necessarily a disadvantage, unless the 
strip MN is excessively wedgeshaped. 
The ellipses obtained are usually long vertically, so that the fringes 
soon become straight and the rotation is extremely rapid whenever the 
center of elHpses is out of the field. It is therefore possible to adjust 
relative to horizontal fringes (parallel to shadow of wire across slit), as 
these incline very obviously for a displacement of less than 10~^ cm. 
and rapidly become vertical. For this reason it makes little difference 
whether the half silvers are on the same or on opposite sides, or whether 
observation be made at T (cd prolonged) or at T' (bd prolonged) . More- 
over the plate MN may be conveniently constructed as in figure 5, of 
two mirrors m, n, attached to the clear strip of plate glass, g, by aid of 
strong steel cHps at c, c' . With the half silvers 5, s' on the same side, 
the wedge angle of the glass is excluded. For shorter diagonals, the 
plan of figure 6 with the silver surfaces 5, s' held together by clips at c 
is preferable. 
If the mirror M\ figure 4, is displaced a distance e where a glass plate 
compensator of thickness E, and refraction constants ju and B is intro- 
duced normally either into ab or hd^ the equation is easily seen to be, at 
wave length X 
E (m - 1) + 2 BjV^ = 2e cos 6 
where 6 is the angle of reflection at M. Using the plate E = 0.434 cm. 
treated above, the first member is 0.2428 cm. Values of e of 0.2420, 
0.2409, 0.2427 were roughly obtained. Hence the mean value of 0 
should be about 60°, as it actually was. Certain outstanding difficulties 
may be met by making MN, figure 4, the short diagonal of the rhombus 
and using the strip figure 6. In such a case 6 at M^ is small and in view of 
the nearly normal reflection at M and N relatively Kttle reflection comes 
from naked glass, sliding is largely avoided and no compensator is neces- 
sary. In this case the fringes for no path difference are actually black 
horizontal fines on a colored ground and far enough apart that 1/10 
fringe could easily be estimated. A test experiment with the above 
plate showed e = 0.1244 cm. corresponding to the small angle 6 a Httle 
over 12°. 
When the U-tube CC, figures 2 and 4, is introduced, the strip M N 
will have to be at a considerable angle (about 45°) to the horizontal, 
so as to raise the N end about 15 cm. above the Mend, corresponding 
to the height of m above m' in figure 2. The new condition however in 
no way changes the general procedure. In case of figure 5 the mirror 
