Royal Astronomical Society. 299 



the assumed east longitude, lO*^ 4'" 6** 25, from the^xaean of all the 

 observations, is + 0'''967 ; -wheiocei )^© rpulting longitude is 

 IQH 4m 7S.217. 



Gn the Use of a new Micrometer, and its Application to the De- 

 termination of the Parallax of Mars at his ensuingj Opposition. By 

 M. Boeruslawski. Communicated, with a Le^er, to'ttle I^reii^'enti 

 from Sir John Herschel. .. 



This communication is introduced ^f/iUki following letlf^r from 

 Sir John Herschel ;— ,. " '" 



ifjq&Db -bum jvjoiQ-iartJ ban «in««ColHngwood, March 30, 1845. q 



•^"^Mydfelirfeli',— ll'feq\*e^''^e¥tAis^ion to lay before the AstrondJ 

 ftScal Society the accompanying communication from M. Bogus- 

 lawski, just received, in which he describes a new micrometer, and 

 urges its application to the determination of the parallax of Mars at 

 his ensuing opposition. ,- .4 



" M. Boguslawski, as will be observed, requests me to take q^ 

 myself the sole charge of directing and arranging the observations 

 he recommends. My engagements utterly preclude the possibility 

 of my undertaking this duty, or any part of it. Nevertheless, being 

 willing and desirous, so far as I am able, to forward this or any 

 other well-recommended scientific object, I have considered it, on 

 the whole, as my duty to forward his letter where it is sure to receive 

 every due consideration ; leaving it to the Council, if it should seem 

 of sufficient importance, to make any arrangements they may see fit 

 for meeting the end in view. . ,. , ■ jsvulv ^-xtU' ;^mi"i 



"I ought to add that I have myself had in' use, for some time 

 during my observations at the Cape, a process of micrometrical mea- 

 surement by means of the transit of stars over the moveable wire of 

 a position micrometer, set at a given angle of inclination to the par- 

 allel, which I have designated as the method of oblique transits for 

 the purpose of determining the distances of double stars, which ap- 

 pears to me nearly or quite identical in principle with this of M. Bo- 

 guslawski. 



" An account of this method, and the results obtained by its use, 



have been for some time written for the press, and will form part of 



my forthcoming observations in the southern hemisphere. — I remain, 



my_,d eajL Sir^c. „ 



___^__ „ J. ^ .^ Herschel." 



..■.\\;-^.-^Z ^^:-v■.^^^.T V>..rr^^^ A.-^ 

 The author states that he intends shortly to publish a detailed ac- 

 count of the construction and use of this micrometer, for observing 

 differences of right ascensions and declinations of stars. Its prin- 

 cipal peculiarity consists in this, that its scale is not dependent upon 

 the individual telescope employed, and is therefore independent of 

 the optical power of it, except only as far as the distinctness and 

 precision of the images are concerned. An account of some obser- 

 vations made with it will be found in some of the forthcoming num- 

 bers of the AstroHomische Nachrichten. It consists simply of one wire, 

 thread, or lamina, which is placed in the common focus of the object- 

 glass and eye-piece of a telescope, as a diameter across the field of 





