1 78 Mr. F. Baily on the Circular Micrometer. 



ment than is perhaps generally known. Three out of the 

 four new planets which have been discovered in the present 

 century, were discovered with very small telescopes, and their 

 motions watched, and the elements of their orbits deduced 

 from observations made with the circular micrometer only : 

 whereby astronomers were enabled to look out for them with 

 certainty, at the time of their re-appearance at the next op- 

 positions. And at the present hour it is almost the only in- 

 strument used on the continent for the observation of comets. 

 To the labours of Olbers, Bessel and Frauenhofer, we are 

 indebted for the high reputation which this little instrument 

 enjoys. To the two former, for the talent which they have 

 displayed in explaining the analysis, by which the observa- 

 tions may be ret! need : and to the latter for the recent im- 

 provement which he has introduced in the construction of the 

 instrument. In a communication like the present, it cannot 

 be expected that I should enter very minutely into a subject 

 of this kind: my object being merely to draw the attention of 

 practical astronomers in this country to an instrument which 

 appears to be very little known here : but which is capable of 

 doing much useful work at a very little expense; and of assist- 

 ing the observations of those who are fortunately possessed of 

 more splendid and powerful instruments. 



By the kindness of Professor Schumacher, who is ever 

 zealous in promoting the interests of astronomy, I have ob- 

 tained one of M. Frauenhofer's circular micrometers, of the 

 most improved and recent construction. This instrument is 

 the same that was exhibited at the last meeting of the Astro- 

 nomical Society of London *; and is called by M. Frauen- 

 hofer the suspended circular micrometer, from the circum- 

 stance of its appearing (in the telescope) as if suspended in 

 the heavens without any support. It consists, in tact, of no- 

 thing more than a circular piece of plate-glass about one inch 

 in diameter, in the centre of which a circular hole is cut, of 

 half an inch in diameter. To the inner edge of this glass 

 circle a narrow ring of steel is firmly and securely fastened ; 

 and, the whole being put in a lathe, the steel ring is turned 

 perfectly circular, and reduced to a very thin edge, both at 

 its exterior and its interior circumference. The glass, with 

 its steel circle, is then burnished into a brass ring or cap, by 

 means of which it may be placed, when required, in the focus 

 of the telescope. 



The advantages attending this construction are, 1° the 

 preservation of the circular form of the ring, as it comes from 

 the lathe, without the risk of its being injured in attaching it 



* See the proceedings of this Society, at the end of the present Number 

 of our Journal. to 



