172 CARNEGIE INSTITUTION OF WASHINGTON. 



OBSERVATIONS WITH THE FIFTEEN-INCH COELOSTAT TELESCOPE. 



In March, 1004. a coelostat of 15 inches aperture was sent to Mount 

 Wilson from the Yerkes Observatory. This instrument had pre- 

 viously been employed by Professors Barnard and Ritchey, of the 

 Yerkes Observatory party, at the solar eclipse of May 28, 1900. in 

 W'adesboro, North Carolina, and by Professor Barnard at the Su- 

 matra eclipse in 1902. As used at Mount Wilson, it is supplied with 

 a second plane mirror, mounted south of the coelostat, and arranged 

 to slide on a north and south track in such a way as to receive the 

 solar rays corresponding to any declination of the sun. 



The ravs are reflected from this mirror toward the north to a 6- 

 inch photographic objective of 61 >4 feet focal length, mounted on 

 the extension of the stone pier just above the coelostat. After passing 

 through this lens the rays traverse a long tube built of wooden 

 framework and covered with paper. The solar image is formed 

 within a small house which terminates this tube at its north end. In 

 the house a photographic plate-holder is mounted, in conjunction with 

 a slide containing a narrow slit, w'hich can be shot at high speed across 

 the solar image by means of a spring. In this way the very short ex- 

 posure required for direct photography of the sun can be obtained. 



One of the chief points of interest connected with this instrument is 

 the efifect of the heating of the air within the tube upon the definition 

 of the solar image. In the first experiments with this apparatus, the 

 skeleton tube was covered on all sides with tar-paper, just as it had 

 been used in the eclipse work. Above the tube, and separated from 

 it by a considerable air-space, was a canvas fly for the purpose of 

 shielding the tube from the direct rays of the sun. It was found 

 that in the early morning, before the tube had become heated, the 

 definition of the solar image was excellent. In a short time, however, 

 heated air within the tube eompletely spoiled the definition, and the 

 sun's image became so blurred and indistinct that no observations of 

 value could be made with it. These circumstances led us to ques- 

 tion what the efifect would be if no tube were employed. The 6- 

 inch lens was therefore mounted in such a position as to throw the 

 beam horizontally through the air toward the north, outside of the 

 tube and over that portion of the ground which was in shadow. The 

 image observed under these circumstances was found to be much 

 better defined than that seen through the heated air of the tube. We 

 accordingly decided to try the experiment of taking ofif all of the 

 paper on the two sides which formed the upper half of the tube. 



