178 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



The simultaneous motion of the strews is accomplished by a small 

 electric motor worked from a distance by the observer (fig. 5). 



The method of observing consists in separating and approaching 

 the first two mirrors and observing the sharpness of the fringes. 

 When they become invisible the distance D of the mirrors M X M 4 is 

 recorded. The theory is not wholly identical with that of the experi- 



^jtf^a-jffi ^- ! -» mi •■ j ) O n ! mm b-(w ^ 



H 



— ' w — nn ' t=i + a h — W 



3^r- 



x^- 



/v 



Fig. 5. 



-Detailed plan of the interferometer. Mi, M 2 , M 3 , M 4 , plane 

 mirrors, mounted on moving carriages. 



ment of Stephan, so that his formula is not applicable. However, 

 the formula for the angular diameter is still 



w=1.22 X/B. 



A readjustment is necessary after each displacement of the mirrors. 

 For easing the labor of this, Michelson has devised various cunning 

 contrivances without which the observations would have been exces- 

 sively laborious if not impossible. 



THE DIAMETER OF BETELGEUSE. 



The decisive observation was made by Pease and Anderson on the 

 15th of December, 1920. The definition was fine and the images were 

 observed without difficuty. The stars (3 Persei, v Orionis, and a Canis 

 Majoris gave fringes with D equalling 306 centimeters. But when 

 the apparatus was trained upon Betelgeuse without changing the dis- 

 tance of the mirrors the fringes could not be seen. At a distance of 

 250 centimeters they appeared, although hazy. It was allowable 

 then to call the value of D equal to 3 meters with an accuracy of 

 about 10 per cent. We thence have 



<o=0".047. 



A restriction is, however, necessary. The theory postulates that 

 the star disk is uniformly bright. It is more probably progressively 



