223° 
id 
and distributions of energy, if the phase-shifting = is only brought 
about by means of cylinder lenses. 
When an approximative method will do for the purpose, a method 
that is only exact for a practically monochromatic beam of one 
: : PAT; Kd : 3 
definite frequency, the phase shifting > Can, indeed, be attained 
without cylinder lenses, viz. by the aid of an oblique-angled paral- 
lelepiped in which the light is totally 
A __reflected. We can now work with the 
unmodified interferometer of Mrcnerson 
(we have only to put the parallelepiped in 
one of the arms, and one of right angles 
—___——}—. - in the other, if we make the paths in 
glass of the same length; fig. 9), but now 
meee we must work with light that has been 
previously polarised either in the plane of drawing or normal 
to it. For sodium light eg., the direction of vibration of 
which is normal to the plane of the drawing, we must have 
< gy = 51°20/21"; then the four phase-shiftings together amount to 
on 
oo | 
that the same parallelepiped could not be used for neighbouring 
spectrum lines too. After some calculations we find namely cos 2p = 
zE (n?—1) cos 674°—1 
1. However p does not so greatly depend on the wavelength, 
: ‚in which n is the index of refraction. 
n 
Dr. Zernike has pointed out another method, in which the phase- 
JT = . 
shifting keeps its value 5 only for one narrow spectrum region, just 
as this was the case with the totally reflecting parallelepiped: If 
we turn the compensator in the interferometer of MicnELSON over a 
small angle and if we place at the same time the movable mirror 
a little nearer, then in the one arm light has to cover a path in 
glass, which is d longer, but in air a path, that is d’ shorter than 
in the other arm. 
If we choose d and d’ so, that they satisfy ud —d’ = — 42 (A), 
even when we vary À (however only over a narrow region) the 
Jt 
phase difference of the interfering beams will be x + 42 Im, when 
1) For * the parallelepiped would assume an impracticable form; index of 
refraction = 1,5153 for light crownglass. 
