532 PEASE— MEASUREMENT OF STAR DIAMETER. 



the mirrors on this side of the interferometer beam separated by an 

 amount indicated by the distance the wedge has been shifted. The 

 angle of the prism is about lo degrees and a linear motion of the 

 prism of i mm. corresponds to a path difference of 0.09 millimeter. 

 If glass be subtracted in moving from the zero fringes to the re- 

 flector fringes, the mirrors must move closer together in order that 

 both sets of fringes may appear in the eyepiece at the same time. 

 The wedge is shifted by means of a rod; one turn of this rod moves 

 the wedge 0.5 mm. and compensates for 0.045 rnm. of air path. 

 Fringes can be observed throughout one-third of a turn of this rod, 

 corresponding to a path difference of about 26 light waves. 



Having in mind the operation of the interferometer and the ap- 

 pearances to be expected, we will turn to the observations made 

 with the telescope in operation. 



Several days were spent by the writer in November, 1920, in 

 preparing the beam for operation, but as several important alter- 

 ations were necessary, actual work was not begun until December. 

 On December 13, with the outer mirrors at 10 feet separation the 

 instrument was put in complete adjustment by observing 13 Persei 

 andy Orionis, both stars known to have diameters much smaller than 

 can be measured with this instrument. This adjustment meant 

 that, upon observing at the eyepiece, both the reference and the 

 interferometer images were seen with fringes superimposed upon 

 them. Upon turning to a Orionis the " zero " fringes were seen 

 but no glimpse could be obtained of the interferometer fringes. 



Turning to a Canis Minoris both sets of fringes were visible si- 

 multaneously, indicating that the instrument was in complete adjust- 

 ment and that the disappearance of the fringes on a Orionis was real. 



It may be thought that reliance cannot be placed upon a null 

 measurement ; there is no reason for this assumption as any in- 

 strumental flexure or atmospheric disturbance requires but a very 

 slight adjustment of the wedge, and at this time the seeing was very 

 good. Dr. Anderson was present on this night and checked the 

 writer's observations. Further observation of a Orionis was not 

 attempted on the succeeding nights in December because the seeing 

 was poor, as was indicated by a reduction in the visibility of the zero 

 fringes; consequently observations were made on a Ceti, a Tauri, 



