209. 
or P" under consideration light falls that has undergone besides 
the phase shifting « resp. an even (among which zero) or an uneven 
number of reflections against denser media. Hence we have to deal 
here with four groups of points, the classification of which will be 
clear from the foregoing. When out of each of these groups one 
point is considered, resp. P, P', P", P"', their vectors make the 
angles cm, cm + a, c'm + a,c'"'m He 42 with the time-direction. 
The phase shifting is taken to be positive if it is a shifting back, 
and negative in the opposite case. When m is now made to change 
its sign, these vectors are now respectively reflected in the time- 
direction ¢, the direction # forming an angle 2 with the former, the 
direction a forming an angle « with the time-direction, and that 
which forms an angle a+ with the time-direction, being a in 
fig. 4. Accordingly the vectors can be divided into two groups, one 
of which is reflected in the line ¢’, the other in a line aa’ forming 
an angle a. As we have put a=, the fan will now probably also 
be transformed, and the intensity will no longer remain the same. 
This is therefore no even function now of the number of waves. 
In such an instrument C and S will accordingly 
t certainly not appear in the formula of intensity in 
a the way of equation (4), which? can again be proved 
0 via the intensity holding for single light. Now the 
separation of C' and S is therefore no longer fun- 
damentally impossible, hence we are on the right 
tE track to determine the function g(x). It is finally 
noteworthy, that a phase shifting in the part LP 
of the light path LPQ will give the same effect as 
one directed in the same way and of the same extent in the part 
- PQ, which remark will appear to be of importance for the inter- 
ferometer of Micueuson. In what precedes it has been supposed that 
the phase shifting «a preserved its value and sign in the reversal of 
sign of m, it must accordingly be a constant or an even function 
of m. Only an instrument in which it is a constant, i.e. independent 
of m, will be of practical use to us. Our purpose is namely to 
examine beams of the most=divergent”’ constitution, and it would be 
impracticable to have to take a different phase shifting into account 
for every kind of light. For this reason we shall e.g. not be able 
to make use of metal reflections. On the other hand it will hardly 
be possible to do without them, for in almost all interferometers 
there occur silvered glass-plates or metal mirrors. This“ difficulty is, 
however, not so serious as it seems. When each of the four mono- 
chromatic beams that illuminated the groups of points P, P', P", PY 
Fig. 4. 
