670 
PHYSICS: C. BARUS 
phenomenon thus depends on the distance apart of two sharp lines of light 
and the interferences are observable before or behind their focal plane. 
5. Reversed Rays. — If the necessary excess of path on the right is 
compensated by a glass column on the left (usually 10 or 15 cm. long), 
the spectro-telescope on adjustment shows a field of concentric half 
ellipses, all terminating in a vertical axis. These interesting phenomena 
are unfortunately not available for measurement as the terminator is 
not sharp enough and as their motion is necessarily sluggish in view 
of the large excess of glass path on one side. The achromatic fringes 
are not producible. For practical work the glass path must be replaced 
by an air path obtained by aid of an of set consisting of two pairs of 
parallel plates at right angles to each other. These not merely com- 
pensate without changing the direction of rays, but on rotating the 
plates as a whole around a horizontal and a vertical axis, they addition- 
ally serve for producing ellipses of any size and of centering them. 
Achromatic fringes are now brilliantly producible. The variety of 
observations made will however have to be given elsewhere. 
6. Plate Method. — In view of certain difficulties encountered in the 
use of reflecting prisms, in particular the loss of rays at the edge, the 
method of figure 3 enlarged in figure 4 was devised. In this the prism 
is replaced by a half silver plate PP' . Hence the rays issuing at 5 and 
reflected by the opaque mirrors at m and n, are thereafter respectively 
transmitted and reflected by the half silvered plate PP' and then reach 
the spectro-telescope at T together. When the path differences are 
sufficiently equal, elliptic interference fringes will be seen in the spec- 
trum. When first found they are usually very fine straight lines; but 
they may be rectified by plate compensators in the beams d and d' or 
mp and np, though the operation is not easy. Leaving these details 
for further consideration elsewhere, the procedure for angular measure- 
ment may advantageously be treated here. For this purpose the half 
silver P and one opaque mirror, n for instance are mounted on a rigid 
bar with an axis at P. The other mirror m is to remain fixed. If the 
bar is now rotated over a small angle a (fig. 4), the mirror at n is dis- 
placed to n' and the ray Sn prolonged (intercept x) is now reflected 
from W to q and thence along T' into the spectro-telescope, parallel to 
its original direction or to the other ray mp. Hence the interferences 
remain intact but many fringes pass during the transfer. The persis- 
tanceoi parallelism is easily seen to be the essential feature of the method. 
To control the fringes either the mirror at n (or at m) may be dis- 
placed on a micrometer screw normal to itself, or the half silver plate 
at P may be displaced parallel to itself. If the angle of incidence at 
n is i and the normal displacement oi n is> e the path difference intro- 
