PEASE— MEASUREMENT OF STAR DIAMETER. 533 



and p Geminorum, with the outer mirrors at 13 feet separation. In 

 February, with mirrors approximately 16 feet and 19 feet apart, 

 observations were made on a Orionis, a Tauri, fi Geminorum and a 

 Bootis. The seeing did not warrant drawing any definite conclusions 

 except that fringes were seen at all points for /? Geminorum. This in- 

 dicates that an interferometer with a base longer than 20 feet will be 

 required to measure its diameter. Fringes were seen for a Tauri at 13 

 feet, at 19 feet, and in March at 14.5 feet, the visibility becoming less 

 with increased separation of the mirrors. Additional measures will be 

 made at points between 16 feet and 18 feet to see whether the 

 fringes disappear as calculations indicate they should. For a 

 Bootis the fringes were much reduced at 16 feet and could not be 

 seen at 19 feet ; the seeing was bad, however, and the observation in- 

 decisive. 



Many stars have been used for checking the instrument ; among 

 them S Tauri, y Orionis, a Canis M:noris,a Geminorum, and iq Bootis. 

 All have shown strong fringes at 19 feet. 



Experience has shown throughout that better seeing is required 

 for this work than was at first supposed, particularly when the 

 mirrors are widely separated and the visibility of the fringes is 

 approaching the point where they disappear. Change of seeing 

 at these times will cause the fringes to flicker in and out, but a 

 check is always at hand, for at the same time the visibility of 

 the " zero " fringes is also reduced. 



On some occasions in bad seeing the zero fringes will remain 

 fixed, but the interferometer fringes will shift to the side of the 

 image, probably because small sections of the wave front become in- 

 clined to the general wave front, due to varying atmospheric den- 

 sities. 



Having determined the distance at which the fringes vanish, we 

 find the angular diameter of the object from the expression 

 a=i.22A/& where a is the angular diameter in radians (206265"), 

 A is the effective wave-length (in cm.) of the star or that portion of 

 the spectrum which is most predominant in forming the fringes 

 seen by the eye of the observer and h is the distance apart of the 

 mirrors (in cm.) ; Anderson has found, in connection with his work 

 on Capella, that the effective wave-length of a solar type star is 



