122 A. A. Atichelson —The relative motion of the Karth 
If, now, the apparatus. be revolved through 90° so that the 
second pencil is brought into the direction of the earth’s mo- 
tion, its path will have lengthened 100 wave-lengths. The to- 
: ee ‘ 8 
tal change in the position of the interference bands would be oa 
of the distance between the bands, a quantity easily measurable. 
e conditions for producing interference of two pencils of 
light which had traversed paths at right angles to each other 
were realized in the following simple manner. 
ight from a lamp 4a, fig. 1, passed through the plane par- 
allel glass plate }, part going to the mirror c, and part being 
1. _ reflected to the mirror d. The 
sexas mirrors c and d@ were of plane 
d glass, and silvered on the front 
surface. From these the light 
was reflected to b, where the 
one was reflected and the other 
refracted, the two coinciding 
along be. 
The distance bc being made 
equal to dd, and a plate of glass 
g being iatebaceed: in the path 
of the ray be, to compensate for: 
the thickness of the glass 0, 
which is traversed by the ra 
e , the two rays will have 
traveled over equal paths and are in condition to interfere. 
The instrument is represented in plan by fig. 2, and in per- 
spective by fig. 8. The same letters refer to the same parts in 
the two figures. 
iy 
y g 
The mirrors c and d were moved up as close as possible to 
the plate 4, and by means of the screw i the distances between 
oint on the surface of b and the two mirrors were made 
approximately equal by a pair of compasses. The lamp being 
