PHYSICS: C. BARUS 
119 
a combination of both spindles and hyperbolae. Probably three layers 
of liquid are chiefly in question, viz., kerosene, kerosene + CS2, CS2, 
and the three stages of form and the sinuous lines correspond to them. 
Fringes were sharp only if viewed in front of the principal focal plane 
of the telescope. 
3. Apparatus. — To obviate the tremor of apparatus which is inevitable 
in the case of the long distance interferometer, the parts were now 
screwed down at short distances in the cast iron block B figure 2. Here 
the ranges MH, HN of half silvered plate, H, and opaque mirrors M, 
N, did not exceed 14 cm., but this gives ample room for the manipula- 
tion x)f^the trough / placed normally in the beam MH. White light en- 
ters_by|way of the collimator SC at any convenient angle 6 (as this does 
not occur in the equations) and 6 = 60° was used. The opaque mir- 
rors M (and preferably also N) are on micrometers with screws normal 
to their faces and each must be provided with adjusting screws relatively 
to horizontal and vertical axes. An elastic fine adjustment (not shown) 
is desirable. The block contains a number of screw sockets, b, for attach- 
ing subsidiary apparatus. The trough, /, should preferably be attached 
to an independent supporting arm, not connected with B and be re- 
volvable about two axes normal to each other. In such a case the posi- 
tion normal to the beam of light may be found from the reverse of mo- 
tion of the interference rings, while the trough is slowly rotated in a 
given sense. 
