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ASTRONOMY: MICHELSON AND PEASE Proc. N. A. S. 
feet (40.8 m.). The fixed position of the inner mirrors gives a constant 
fringe spacing equal to 0.02 mm., which is easily visible with a power of 1600. 
Coincidence of the two interferometer pencils at the focus is produced by 
adjusting the inner mirrors during the day and the outer mirrors on a star 
at night. Equality of path is obtained by placing the outer mirrors sym- 
metrically on the beam, as nearly as possible, and then adjusting a double 
wedge of glass lying two feet within the focus in the path of one of the 
pencils. 
FIG. 1 
Compensation for the mean thickness of the wedges is made with a plate 
of plane-parallel glass placed in the other pencil. 
One of the wedges is fixed, the other is adjustable in a direction par- 
allel to the inclined surfaces, a linear motion of 1 millimeter introducing an 
equivalent air path of about 0.09 mm. Fringes can be observed through- 
out a linear motion of the wedge amounting to 0.16 mm., corresponding to 
about 26 light waves. This range can be increased by introducing a 
direct- vision prism behind the eyepiece. 
For comparison purposes a series of reference or "zero" fringes is ob- 
tained in the eyepiece by covering the end of the telescope tube completely 
save for two apertures in the beam (in addition to those of the inner mir- 
rors) 6 inches in diameter. 
When the interferometer is in perfect adjustment and the outer mirrors 
are close together, two separate star images are seen in the eyepiece, one 
formed by light reflected by the mirrors, the other by light admitted by the 
supplementary apertures which produce the zero fringes. Each image is 
surrounded by diffraction rings and crossed by the interference fringes, 
