270 PROCEEDINGS OF THE AMERICAN ACADEMY 



XIV. 



ON A MECHANICAL ATTACHMENT FOR EQUATORIAL 

 MOUNTINGS, TO FACILITATE SWEEPING IN RIGHT 

 ASCENSION. 



By D. P. Todd, M.A. 



Presented May 12, 1880. 



Not infrequently it happens that the astronomer has occasion to search 

 a portion of the heavens defined in right ascension and north polar dis- 

 tance. The general method of such searches consists in a subdivision 

 of the entire area into a number of zones, — of a convenient length in 

 right ascension and of a width in declination somewhat less than the 

 field of view of the eye-piece. No special difficulty attaches to the 

 mere shifting from one zone to another in declination : this may be 

 done quite automatically by a known amount of rotation of a tangent- 

 screw applied to the declination-circle ; or the observer, watching some 

 star that happens to be in the right part of the field, can turn the tan- 

 gent-screw until the instrument points upon the new zone. 



To define the limits of the several zones in right ascension, however, 

 is not so simple a matter. If it is not important that the limits of the 

 zones be accurately observed, and neighboring stars are readily visible, 

 perhaps the observer may get along fairly well by simple eye-align- 

 ment. Or, if he has an assistant at the right-aseension-circle, he can 

 be duly apprised of the termination of the zones. Or, each sweep in 

 right ascension may be terminated quite at random, the telescope being 

 moved so far each time that the entire zone shall lie surely covered: 

 there must, nevertheless, be frequent reference to the clock and 

 circle. 



All of these methods take a deal more time than is employed in the 

 actual search at the eye-piece. If, without leaving the eye-piece, the 

 observer had some convenient way of knowing the moment when his 

 telescope had reached the end of the zone, much of his time would be 



