abbot] new ccelostat and horizontal telescope 89 



servatory, in connection with the long-focus mirror above men- 

 tioned. There is also shown in the illustration a portion of the 

 " churned " tube of the horizontal telescope, of which more will be 

 said later. The ccelostat carries a thirty-inch and a twenty-five-inch 

 mirror, the former turned by a polar axis driven at the rate of one 

 complete rotation in forty-eight hours, the latter mounted on a car- 

 riage with traverse motions at right angles like the slide rest of a 

 lathe. The cell of the second mirror is carried by trunnions in a fork 

 itself capable of turning about a horizontal north and south axis, and 

 by these two motions of rotation, with their fine adjustments, the 

 beam may be sent in any direction whatever, though most favorably 

 in a nearly northerly one. In actual use the reflected beam is de- 

 pressed about 6° from the horizontal to feed the long-focus mirror,^ 

 which is 55 feet north and about 3^ feet below the center of the first 

 mirror of the ccelostat, directly under which the beam passes toward 

 a focus on the third pier, some 85 feet further south. To provide for 

 this depression of the beam from the horizontal, the north and south, 

 or declination, track of the ccelostat is inclined upward at a corre- 

 sponding angle, so that the reflected beam may always clear the first 

 mirror. The length of travel of the lower base of the second mirror 

 on this north and south track is five feet and the lower base itself 

 has an east and west track six feet long on which the upper casting 

 is moved to and fro to allow for avoiding the shading of the main 

 ccelostat mirror by the cell of the twenty-five-inch mirror between 

 II o'clock and i o'clock near the times of the equinoxes. 



THE TUBE AND STIRRING DEVICE 



Early experiments on an artificial star with the long-focus mirror, 

 before the completion of the ccelostat or the installation of a tube, 

 showed conclusively that the " boiling " caused by irregularities of 

 the atmosphere over the grass-grown soil between the mirror and its 

 focus was far too great to permit anything like satisfactory defini- 

 tion on the solar image, and therefore the novel device of a 

 tube with provision for stirring the air by means of a blast was 

 ordered. It consists of a main horizontal tube 24 inches in in- 

 ternal diameter with diaphragms at five-foot intervals, and with 



' If the telescope was intended for stellar or lunar photography it would 

 not be allowable to use the concave mirror quite so far out of axis as the 3° 

 thus required; but the deformation of images from this cause, as computed 

 by the formula of Poor {Astrophysical Journal, \ii, 120, 1898) for the 

 long-focus mirror is only about o".2S of arc, which is inappreciable compared 

 with other defects of solar definition. 



