1902.] SXYDER— A NEW METHOD OF TRANcSlTING STARS. 203 



berg Observatory, where its Director, H. Struve,^ has successfully- 

 applied clockwork directly to the Repsold micrometer, and thus 

 unquestionably improved its usefulness. With this work the writer 

 became acquainted only after his own plan had been consummated 

 and the resulting instrument constructed and mounted for use. Dr. 

 Cohn,^ of the same Observatory, has recently published an extended 

 investigation which shows marked advances in accuracy over the 

 usual methods of observing. Struve's method has, however, thus 

 far involved the unsymmetrical placing of the weight of the appa- 

 ratus and, while itself possessing serious mechanical limitations, 

 does not avoid certain peculiarities and limitations of the Repsold 

 micrometer. The necessity therefore still exists for a method that 

 shall be flexible in adaptation and use, and not impose unreason- 

 able conditions on the observer. 



The conditions to be attained in an effective method were early 

 formulated by the writer substantially as follows : 



The ordinary micrometer of a transit instrument shall be used, 

 and its movable wire driven electrically at approximately uniform 

 speed. The rate of driving shall, as required, vary with the decli- 

 nation. The direction of motion shall be instantly reversible. 

 The wire shall be promptly readily started on its course when bi- 

 section of the star occurs. While in motion the wire shall be 

 easily regulable for bisection of the star. The automatic chrono- 

 graphic record shall be made at whole turns or at fractions of a 

 turn of the screw as desired. 



In practically studying the electrical method of determining and 

 controlling the motion of the thread of a transit micrometer, it has 

 been found that there are three principal plans of adaptation 

 available : 



I. A small electric motor may be placed on or near the head of 

 the transit instrument, with its axis parallel to that of the instru- 

 ment. The varying rate of motion required for change in decli- 

 nation may then be secured by regulating the field of the motor 

 and, if necessary, also that of a small dynamo supplying the cur- 

 rent. The main difficulties in this plan are, the wide range of 



1 H, Struve, " Ueber die Verbindung eines Uhrwerks mit dem unperson- 

 lichen Mikrometer von Repsold," Astron. Nach., 3719, 1901, Marz. 



2 Dr. Fritz Cohn, ** Ergebnisse von Eeobachtungen am Repsold'schen Regis- 

 trirmikrometer bei Anwendung eines Uhrwerks," y/^/rc>«. A'<3r/^, 3766-67, 1901, 

 November. 



