PHYSICS: C. BARUS 
337 
Since the value of AiV is within 10~^ cm. and fl" = 10 cm. in the above ap- 
paratus, we may further write at mean temperatures (25°) a = 2.5 X 10"*; 
M = 1.333; B = 10-11 x 3.1, 2B/\^ = 0.018 at the D Hne. Thus 
M - 1 + 2J5/\2 = .351 and A/ = 10-V3.51 X 2.5 X lO"* X 10 = 0.114°. 
In other words in case of tubes 10 cm. long, the effect of a difference of 
temperature of about 0.1° between the tubes should be easily observable 
by mere displacement, whereas a difference of less than 0.03° would 
be equivalent to the passage of one interference ring. 
Again from equation (2), if = 10"^ cm. then AH = 10-V.351 = 
10-* X 2.8 cm. or about 9 X 10"^ cm. per vanishing interference ring are 
the displacements to be anticipated. These are equivalent to pressures 
of about 0.3 and 0.1 dynes per cm^. 
A number of experiments were made with this apparatus but for these 
there is no space here. 
5. J amines Interferometer. — The ease with which the Michelson inter- 
ferometer may be adjusted and its remarkable adaptability have led 
to its general preference over the older form of Jamin. Nevertheless 
the latter furnishes two parallel rays which for such purposes as the 
present are desirable. Hence if the four faces of the interferometer be 
separated in the manner suggested by Mach, a very available form of 
interferometer is obtained. But the trouble with the arrangement is 
the difhculty of adjusting the four surfaces. Not only are the centers 
of ellipses liable to be remote from the center of the field, but it is often 
hard, without special equipment, to even find the fringes. 
If however the device which I suggested elsewhere is adopted, i.e., if 
figure 4, the half silvered plates, M, N, are at the ends of a simple strip 
of plate glass, so that rays terminating in M M' N iV' after adjust- 
ment necessarily make a rhombuslike figure symmetrical to MN the 
fringes are found at once: for they appear when the white sKt images 
in T coincide horizontally and vertically and the rays bd and cd inter- 
sect in the conmion point d. Hence the mirrors, M\ N\ should be on 
carriages D, F, adapted to move on the parallel sHdes S, S\ M, N 
may also be put on a carriage E though this is not necessary. S, 5" 
need not be parallel to ac or bd. If the mirror and are wide, 
considerable latitude of adjustment is thus obtained. 
If if iV is half silvered on the same side (i.e., toward iV^a compen- 
sator is needed in ac or cd, if path difference is to be annulled (symmetry). 
If however M is half silvered on the side and N on the side, no 
compensator is required. In the latter case however, if ordinary plate 
glass is taken, M and N are not quite parallel and the elKpses will be 
