212 
when the rays 1 and 2 from the points A and B lie in the same 
horizontal plane, and SA and SB make equal angles with the radius 
SC, which passes through the optical centres. The rays 1 and 2 
being coherent, the components 17 and 2d e.g. can interfere with 
each other. The leaving rays can therefore interfere in pairs, and 
as the rays which form such a pair, run immediately by the side 
of each other, a sharp interference image will be formed in the 
focal plane of the objective. Now tbis is not looked at, as usual, 
with the ocular as magnifying glass, but the ocular must form a 
real enlarged image of it, which falls outside the telescope, and of 
which only the central part will be used by us. All the light that 
has entered the prisms at right angles, will be focussed in one 
point Q'. 
In order to calculate the intensity in the point Q’, it is convenient 
to introduce a reference plane HA, determined by the following 
equations: 
ED == CO LPN FE LM Ne 
which quantities refer to geometrical lengths, and not to optical 
lengths. 
Also the two following remarks will greatly facilitate the calcu- 
lation of the phase-difference between the interfering beams. 
1. Let the distance from the front plane to the opposite side be 
p for the immovable prism, and equal to q for the movable prism; 
then every ray that strikes the front plane at right angles vertically, 
covers a path in the prism of the length 2p or 2q, according as 
this takes place in the immovable or in the movable prism. 
2. The system of lenses makes a plane wave again to a plane 
wave, so that in fig. 5 the optical distance from A to A’ is equal 
to that from C to C’. Accordingly in fig. 7 the system of lenses 
will retard the light as much as a glass plate of the thickness 2d, 
when the thickness of every lens in the middle amounts to d. Besides 
Jt 
this retardation the system of lenses causes the phase-shifting — 
forward, which on the other hand means an acceleration. In order 
to get from C over the immovable prism to U, the light must pass 
three times through the glass plate, hence cover a path 3 d in glass, 
and further the paths of the following lengths in the following media: 
DE in air, 2p in glass, (FG—2d) in air, and 2d in glass. 
The light that reaches the point U from C across the movable 
prism, must also pass three times through a glass plate, hence it 
must again cover a distance 3d in glass, and further the following 
