A BRIEF DESCRIPTION OF THE ONE HUNDRED-INCH 



REFLECTING TLESCOPE OF THE MOUNT 



WILSON OBSERVATORY 



By Professor Walter S. Adams, D. Sc. 

 Mount Wilson Astronomical Observatory 



With the introduction into astronomy of the instruments 

 used in the phj^sical laboratory for analyzing light sources and 

 studying" their brightness and mode of radiation the telescope 

 has come to be regarded mainly as an instrument for collecting 

 light. The physicist to a certain extent has the light source at 

 his control, but the light of a star is a fixed quantity, and the 

 only way in which the astronomer can increase the brightness of 

 the image which he desires to examine or to analyze is to 

 increase the aperture of his telescope. One instrument of twice 

 the diameter of another will collect four times as much light, 

 and will form an image of a star four times as bright, other 

 things being equal. At a period in astronomy when powerful 

 spectroscopes are being employed for studying the motions and 

 the chemical constitution of stars, and when the problem of the 

 structure of the universe requires that we discover and deter- 

 mine the brightness of as many as possible of the faintest stars 

 in the heavens, the value of a great telescope is obvious. 



The project of the 100-inch reflecting telescope took form 

 in 1906, when Mr. John D. Hooker provided the funds for the 

 purchase of a suitable disk of glass, the erection of a building 

 for the necessary optical work, and the employment of skilled 

 opticians to fig'ure the surface of the mirror. In the winter of 

 1908 a disk was received from the St. Gobain Glass Company 

 of France. On examination it was found that owing to the 

 method in whicli molten glass had been poured into the mould 

 a considerable number of particles of unmelted material and of 

 air bubbles had found their way into the interior of the disk. 

 Although in the case of a reflecting telescope the light does not 

 enter the glass at all. and the mirror material may as well be 

 opaque as transparent, it was feared that strains might be 

 present in the body of the glass which would prevent the mainte- 

 nance of a perfect optical figure under varying conditions of 

 temperature. Accordingly no attempt was made to utilize this 

 disk, and further trials were begun by the St. Gobain Company 

 to secure a fully transparent block of glass. These, however, 

 were unsuccessful, and after considerable delay it was decided 

 to have recourse to the original disk. Before the extensive 

 work of figuring the mirror was carried far, tests were made 

 at varying temperatures and in dift'erent positions of the disk 

 to determine the existence of strains or flexure. No such effects 

 were found and the optical work was continued. The mirror 

 was finally completed in the summer of 1916. During this long 



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