NO. 1865 OUR knowledge; of the sun HALE 339 



•Observatory, with the aid of funds given by Miss Snow, of Chicago, 

 had its first trial shortly before our work on Mount Wilson was 

 undertaken. It was afterwards brought to California in connection 

 with an expedition sent out by the Yerkes Observatory, with the 

 aid of a grant from the Carnegie Institution, and was ultimately pur- 

 chased by the Mount Wilson Solar Observatory as a part of its 

 permanent equipment. 



This instrument is designed to give a sharply defirted image of 

 the Sun, nearly 7 inches in diameter, at a fixed position within a 

 laboratory, where its various details can be investigated with spectro- 

 scopes or spectroheliographs of any desired dimensions. The coelostat 

 shown in Plate xxvi carries a mirror 30 inches in diameter, mounted 

 so that the plane of its front (silvered) surface is exactly parallel 

 to the Earth's axis. When this mirror is rotated by a driving-clock 

 at such a rate that it would complete a revolution in forty-eight hours, 

 a beam of sunlight reflected from it is maintained in a fixed position, 

 in spite of the apparent motion of the Sun through the heavens. 

 This beam falls upon a second silvered mirror, 24 inches in diameter, 

 which sends the rays toward the north. Both of these mirrors have 

 •optically plane surfaces, and their function is merely to bring the 

 Sun's rays into the telescope house and to direct them upon a con- 

 cave mirror 24 inches in diameter, mounted 95 feet north of the 

 coelostat. This mirror, which may be regarded as the telescope 

 proper, returns the rays 60 feet toward the south to a point just 

 outside of the entering beam, where it forms an image of the Sun 

 nearly 7 inches in diameter. By setting the concave mirror at the 

 proper angle, the solar image can be made to fall upon the slit of 

 a spectrograph of 18 feet focal length, or upon the slit of a large 

 spectroheliograph. Both of these instruments are mounted on mas- 

 sive stone piers. Thus all restrictions as to the dimensions and 

 weight of such auxiliary apparatus are removed. 



The house in which the Snow telescope is mounted (Plate xxvii) 

 was designed with the object of keeping the temperature of the air 

 within it as nearly as possible the same as that of the outer air. It is 

 constructed of a light steel framework covered with canvas louvers 

 and provided with a ventilated roof. Without such precautions the 

 air within the house would become heated during the day, and the 

 difference in temperature between the inner and outer air would 

 cause distortion of the solar image and consequent blurring of its 

 details. In practice, on day after day in the summer months, the 

 image of the Sun given by the telescope during the early morning 

 hours is nearly as clear and distinct as a steel engraving. 



