364 PROCEEDINGS OF THE AMERICAN ACADEMY 



XX. 



LAEGE TELESCOPES. 

 By Edward C. Pickering. 



Presented April 13, 1881. 



The small amount of work accomplished with large telescopes has 

 often been the subject of unfavorable comment. This criticism ap- 

 plies with especial force to this country, where there are nearly a 

 dozen telescopes having an aperture of a foot or over, besides two of 

 the largest size now in course of construction, and two of 26 and 24 

 inches aperture which are unmounted and have been for several years 

 perfectly useless. Among so many, it seems as if one might be spared 

 for a trial of the following plan, which if successful would produce at 

 a small expense far more work than could be obtained with a mount- 

 ing of the usual form. 



Suppose that the telescope is placed horizontally at right angles to 

 the meridian, and that a plane reflector inclined to its axis by 45° is 

 placed in front of it. This reflector may revolve around an axis coin- 

 ciding with that of the telescope. Such a mounting has been used in 

 transit instruments, and gives much satisftiction in the meridian photo- 

 meter of the Harvard College Observatory. The principal difficulty 

 with a large instrument would lie in the flexure of the reflector. This 

 difficulty has, however, been overcome in a great measure in reflecting 

 telescopes by various ingenious devices. In the present case, since the 

 reflector rotates only around one axis instead of two, the problem 

 is much simplified. A slight motion at right angles of perliajjs 5° 

 would be a great convenience, as will be sliown below, and would 

 probably be insufficient to materially affect the flexure. It may be 

 said that it is more difficult to make a plane surface than one that is 

 curved. But the principal effect of a slight curvature would be to 

 change the focus of the telescope, the aberration being much less than 

 the effect of the varying flexure. Let us admit, however, that tlie best 

 definition cannot be obtained, in considering the purposes to which 

 such an instrument could be api)lied without disadvantage. 



