PEASE— MEASUREMENT OF STAR DIAMETER. 



529 



the ratio of A/a is 1.22. As the slits are still further separated the 

 fringes reappear and disappear several times, but are always increas- 

 ingly weaker. 



VI5I51UTY OF FRm6E5 FOR 

 VARYlNa DISTANCE BETWEEN 5UT5 



CIRCULAR D15^ 



DISK PAR^ENE\) AT EDGE 



RELATIVE. PI STANCE BETWEEN 5L1T5 



Fig. 4. Visibility curves. 



It has been found for the sun that the brightness of its surface 

 is not uniform and that it fades away towards the limb. The dotted 

 line represents the manner in which its visibility varies as the dis- 

 tance between the slits is changed, and indicates that a factor of 

 1.33 is to be used, instead of 1.22, for a type of star similar to the 

 sun. 



There is no known star for which total disappearance of the 

 fringes can be obtained with the aperture of the 100-inch reflector; 

 a Orionis, as mentioned above, presented only a falling ofif of in- 

 tensity at the extremities of the diameter of the mirror. 



Accordingly, for the actual measurement of star diameters 

 Michelson and Pease designed a beam (Fig. 5) to carry the four 

 auxiliary mirrors, following Michelson's plan of 1890. This is 

 placed upon the end of the Hooker telescope and the observations 

 are made at the Cassegrain focus, which has an equivalent focal 

 length of 134 feet. The pencils of light from the star are reflected 

 from the two outer mirrors, which are adjustable, to two inner 



