530 PEASE— MEASUREMENT OF STAR DIAMETER. 



mirrors. They then follow the ordinary path in the telescope, 

 first to the great mirror, thence to the convex mirror, finally to a flat, 

 which for convenience of observation brings the pencils to the 

 focus at the side of the instrument. 



The bed of the cross-beam consists of two lo-inch steel channels 

 21 feet long with their flanges turned inwards, separated by sections 



Fig. 5. Twenty-foot Interferometer Beam on 100-inch Reflector — Diagram 



of Light Paths. 



of 12-inch channel and boxed on the lower side with 0.187-inch 

 steel plate, all riveted securely together. Openings were cut where- 

 ever possible to decrease the weight and the inner edges of the top 

 flanges were planed true to o.ooi inch, the frame being supported in 

 the position in which it was to be mounted on the telescope. 



The four mirrors are 6 inches in diameter, inclined at an angle 

 of 45°, the outer ones facing upwards, the inner ones facing down- 

 wards ; they are mounted on slides and the two outer ones, thus far 

 moved by hand, are being equipped with screws driven by a single 

 motor, to keep them equidistant from the inner mirrors. 



Since the inner mirrors are fixed 45 inches apart, the spacing 

 of the fringes at the focus is constant and equal to 0.008 inch. 

 Experience has shown that fringes of this size can be conveniently 



