DIAMETERS OF THE STARS DAN JON. 177 



technique. Nevertheless many physicists might have hesitated, 

 despite this simplification, at the idea of causing two bundles of 

 rays to interfere which at their start were situated 5 or 10 meters 

 apart. For approaching this startling feat, the audacity of the 

 American physicist, his profound knowledge of optics and long 

 experience with the most delicate apparatus was necessary. 



The first interferometer, constructed in 1920, had the two receiv- 

 ing mirrors 6 meters apart. They were mounted upon the Hooker 

 telescope (fig. 4), not because of its optical power, which was not 



Fig. 4. — The 2.50 meter telescope provided with the 

 interferometer. 



The light, reflected by the mirrors, Mi, M 2 , M s , and 

 M 4 of the interferometer upon the great parabolic 

 mirror a, is then reflected to the hyperbolic mirror 6 

 (the telescope is mounted as a Cassegranian). A plane 

 mirror c, inclined at 45°, then sends the light to the 

 eye-piece d situated at the side of the tube. It is close 

 to this ocular that the compensating glasses are placed. 



necessary, as we have just seen, but because of the advantages which 

 accrued from its massive and rigid equatorial mounting, which 

 admitted the extra load without accompanying trouble. Another 

 interferometer of 1G meters, now actually under construction, will, 

 on the contrary, be an independently mounted instrument. 



We will give some details of the 6-meter instrument. The mirrors, 

 15 centimeters in diameter, are carried by a trussed girder. The 

 two central mirrors are mounted 1.14 meters from each other. The 

 other two can be reciprocally displaced. In order to accomplish 

 this their supports are mounted on slide rests moved by two screws. 



