211 
make use of two cylinder lenses, whose focal lines coincide; in fig. 
5 they are thought with the descriptive lines normal to the plane 
of the drawing, and we shall assume this position in all the follow- 
ing figures. Lines thought above or in the plane of the drawing 
will be drawn in full, whereas lines that are supposed below it, 
will be dotted. Rays recurring on the upper side parallel to the 
plane of drawing (e.g. a), can never be brought to the lower side 
by the cylinder lenses, because 
the lenses have no refractive power 
in the direction of their descriptive 
lines. Analogously for rays 6 under 
Fig. 5. and parallel to the plane of the 
drawing. The cylinder lenses will have to be of such a quality that 
they do not disturb the phenomenon of interference. 
It is not possible to place this set of cylinder lenses without any 
modification into one of the arms of the Micur.son interferometer, 
because the light then passes twice through the system and the 
S resulting phase-shifting becomes 
again st. Compare fig. 6, in which 
the immovable and the movable 
mirror are the two halves of the 
instrument consisting resp. of the 
points P" and P; the system of 
lenses now lies as well on the 
path LP" as on the path P"Q, 
and we saw that phase shiftings 
are equivalent in the two parts of 
Q the light path, when their direction 
Fig. 6. is the same, e.g. when both are 
directed forward as here, which causes them to join to 2. For our 
purpose the inferometer of MicueLson must, therefore, be modified. 
Fig. 7 indicates how this can be done. 
The mirrors of Micne.son’s interferometer have been replaced by 
totally reflecting prisms, through which the advantage is attained 
that the light goes only once through the lenses. As the two beams 
undergo precisely the same total reflections, we need not take into 
account the phase-shiftings appearing in this. When a ray which 
has originated from a ray originating from the slit S on its reflect- 
ion on the silvered glass plate through reflection or transmission is 
resp. provided with the index r or d, the figure shows how the 
reflected component 17 of the ray 1 leaves the apparatus iinmediately 
by the side of the transmitted component 2d of another ray 2, 
