III. 



On the Square Bar Micrometer 



By S. C. CHANDLER, Jr. 



Communicated October 14, 1885. 



1. The use of a square for the micrometrie observation of comets was proposed 

 in 1800, by D. Burckhardt, in Zacli's Monattiche Correspondent , Vol. I. p. 120. The 

 idea had been suggested to him by a figure of such an instrument in Adam's 



tt Description of Mathematical Instruments." Burckhardt shows the simple manner 

 in which, by means of transits over the sides and the diagonal of the square, 

 the difference of position of two objects can be determined, in any position of 

 the square with reference to the diurnal motion, and its advantage over Bradley's 

 rhomboidal micrometer in this respect. Nearly half a century later, the square 

 was independently proposed by Mr. A. Graham, at the Markree observatory, and 

 a very large use of it was made in the determination of approximate places of 

 stars, for charting purposes. Graham constructs his micrometer by four flat bars 

 of thin steel, laid over each other in the form of a square, and fastened by their 

 projecting ends to a circular diaphragm. An improvement in the construction is 



to have a similar figure to that formed by the overlaid bars and the diaphragm 



cut out of a single plate of thin metal, so that all four sides lie in the same 

 focal plane. Such a micrometer has been in use for about twenty years at the 



Harvard College observatory. 



It seems singular that the square bar micrometer has not come into more 

 general use. It appears to me to be preferable to the ring, having over the 

 latter two important advantages, without comoensating defects. First, all the 



comp 



transits may be observed over the folio wing edges f the bars, so that the entries 



o ~~n 



and exits are both determined by the phenomenon of disappearance; consequently 

 the lengths of the observed chords are largely free from that highly prejudicial 



source of error, which is so troublesome in the ring micrometer, arising from the 

 difference in estimating transits of such dissimilar objects as stars and comets, 



