'EASE— MEASUREMENT OF STAR DIAMETER. 531 



examined with an eyepiece of one inch focus. After the beam is 

 placed on the end of the telescope the inner mirrors are first ad- 

 justed in the daytime by mounting a miniature lamp at the focus and 

 observing its position, after reflection through the entire mirror 

 system, as seen projected in the outer mirror when viewed from 

 above the latter. Adjustment is made until the lamp apparently lies 

 central in the outer mirror. At night a bar 45 inches long is placed 

 across the fiat-mirror mounting near the focus, centrally across the 

 axis and then by sighting upward, the outer mirrors are adjusted 

 until the end of the marker is near the center of the small region over 

 which the star can be seen reflected in the inner mirrors. Observa- 

 tion at the focus now shows the star image lying somewhere within 

 a. field 20 inches in diameter, and further adjustment of the outer 

 mirrors places the two beams centrally in this opening and in coin- 

 cidence with each other. 



To obtain a reference or zero image, the entire end of the tele- 

 scope is covered save for two 6-inch openings, in addition to those of 

 the interferometer mirrors, and the two admitted pencils pass over 

 the ordinary course in the telescope and form an image at its focus. 

 Usually the interferometer image is brought within a quarter of an 

 inch of this image and both are viewed in the eyepiece simultane- 

 ously. 



To obtain interference fringes the optical distance from the star 

 to the focus along the two pencils must be the same. Experience 

 has shown that it is very difficult to obtain this equality by simple 

 motion of one of the mirrors along the beam. Compensation is 

 accomplished by placing, about two feet within the focus, a double 

 wedge of glass in the path of one pencil and a plane parallel plate 

 of glass equal to the mean thickness of the wedges in the other 

 path. The outer mirrors are first set equidistant from the inner 

 mirrors by steel scales, then, observing at the eyepiece, one of the 

 wedges is slowly shifted so as to increase or decrease the equivalent 

 glass thickness until a point is reached at which the fringes appear. 

 As light travels faster in air than in glass we must add glass if the 

 path is relatively too long or take it out if too short. If glass thick- 

 ness must be added in moving from the zero fringes to the reflector 

 fringes, the path is too short ; to equalize the wedge is backed off and 



PROC. AMER. PHIL. SOC, VOL. LX., U, MARCH. 20, ig22. 



