362 
PHYSICS: C. BARUS 
2. Displacement Interferometer. Jamin Type. — These considerations 
induced me to devote further study to the Jamin type of interferometer, 
figure 1. The mirrors ikf, N' were put on one pair of long slides (1.5 
meters long) parallel to ac and the mirrors M' N on similar slides paral- 
lel to the former. In this way any distance ac or bd was available. 
The beams were about 16 cm. apart corresponding to a normal dis- 
tance between the end mirrors {N N' , M M') of about 12 cm. But 
these distances could also be increased from nearly zero {M and M' 
nearly contiguous) to about 20 cm. in view of the width of mirrors used. 
The angles at a,b,c,d were each about 45°, so that a rectangle of rays 
is in question. 
The adjustment proved eventually to be quite simple by aid of a 
horizontal beam of sunUght with weak condenser lens and collimator. 
A thin wire is to be drawn across the slit. M and are first set for 
parallelism in the absence of N and by adjusting the images of the 
slit at the same level (horizontal) and equidistance on a distant wall. 
The mirrors N' and are next put in place with the distances acd and 
abd about equal. The two images seen in the telescope at T {g re- 
moved) are then made to coincide both horizontally and vertically by 
adjusting N and and these are then sHd by a small amount on their 
slides (direction ac) until the rays are coincident at d to the eye (light 
strips on the mirror coincide). 
If now the grating g is inserted very, fine oblique fringes will usually 
be seen. These may be enlarged to a maximum by moving the microm- 
eter controlling the displacement normal to itself. Somewhat 
coarser horizontal Hnes are thus obtained. 
Finally the distant centers of the ellipses are brought into the center 
of the telescope by aid of the thick glass compensator (figure 2), (the 
equivalent air path of the other ray being correspondingly lengthened), 
by rotating the glass plate on a horizontal axis. The same result may 
be obtained by rotating N and N' on a horizontal axis, successively by 
small amounts, into parallelism with M and M\ But the compensator 
is more convenient. 
The ellipses so obtained with common plate glass and a film grating 
at g were magnificent. A rough test of the diplacement interferometer 
was made by using the above plate glass of thickness E = 0.434 cm. 
where z = £ (m- 1) + 2 J^/X^ = 0.2428 cm. In two experiments 
agreeing to within 10~^ cm., 2e = 0.3448 cm. were the displacement 
obtained. Assuming that 0 = 45°, 2e cos d = 0.2438 cm. This agrees 
with z as nearly as may be expected unless 0 is specifically measured. 
3. Broad Slit Interferences. White Light. Residual Fringes. — Some 
