476 
PHYSICS: C. BARUS 
It is thus an essential extension of the same method as used for reversed 
spectra, heretofore, and also of the methods in which the paths are 
essentially small. 
In figure 1, P is the first prism cleaving the white beam, L, diffracted 
by the slit of the collimator. M and N are the opaque mirrors, the 
former on a micrometer. For greater ease in adjustment, the second 
prism is here right angled, though this is otherwise inconvenient, 
since the angle, 8 = 90°- (p, is too large. The rays reflected from 
impinge normally on the reflecting grating G (grating space, D = 200 
X 10-^) and are observed by a telescope at T. P, P', M and N are all 
provided with the usual three adjustment screws. must be capable 
of being raised and lowered and moved fore and aft. The field is bril- 
liantly illuminated. When the path difference is sufiiciently small 
the fringes appear and cover the whole length of superposed spectrum, 
strongly. They are displaced with rotation if M is moved normally 
to itself. 
As first obtained the fringes were too close packed for accurate meas- 
urement; but the following example of the displacement, e, of the mirror 
