GREAT REFRACTING TELESCOPE AT ELCHIES, IN MORAYSHIRE. 379 



A, 7. — Proper Motion Effects. 



Though the list of stars observed at Elchies was small, yet it contained not a 

 few instances where previous observers were more or less uncertain, whether the 

 group should be classed as a physical and real, or only as an optical and unreal 

 or apparent, double star. Some of these cases have been affirmatively settled by 

 the Elchies micrometrical observations exhibiting proofs of orbital movement, 

 and others by the test of " proper motion." This latter, however, being applied 

 in rather an extreme manner, I propose to introduce at this point a tabular view 

 of the data, and trust that it will be found a convenient and important reference 

 throughout the next section of the paper. 



The " proper motions" employed, are those of the British Association Cata- 

 logue, where they are given in their annual amounts, generally for the larger 

 component only of each star group. If in any particular instance, the smaller 

 members of the group form a physical system with the larger, evidently they will 

 experience the same amount of proper motion, — whether real from their own 

 movement, or apparent from our sun's, — and their relative " positions and dis- 

 tances" will not be altered thereby. But very different results will follow, if 

 the members of any star-group are not physically connected together, and only 

 the larger component be endued with the British Association Catalogue recorded 

 amount of Proper Motion ; and if the accumulated effects of such motion can 

 be traced through a considerable period of time. Now, the greater part of the 

 " Cycle" references being about thirty j'^ears earlier than the Elchies work, I 

 have multiplied the British Association proper motions by that number, and 

 then computed their effect in altering the relative positions and distances of the 

 members of each star- group concerned, on the hypothesis of their being in 

 apparent proximity to each other only. 



The result is usually an alteration so greatly exceeding the bounds of probable 

 error in an Elchies observation, that, according as it, the alteration in position 

 and distance, is or is not borne out and reproduced by, and in, these observations, 

 we are enabled to state with great confidence (subject only to the truth of the 

 said British Association Catalogue proper motions), whether the group belongs 

 to the one or other leading variety of double stars. 



wire. The value of the micrometer screw in seconds of space was derived from transits of the pole- 

 star, measured with a sidereal pocket chronometer, kindly lent by Dr Lee of Hartwell to the Edinburgh 

 Royal Observatory for many years, and brought northwards on this occasion. 



VOL, XXIIL PART IL O L 



