VOL. LXXI.] PHILOSOPHICAL TRANSACTIONS. 155 



nified much beyond what its light would admit of, appeared hazy and ill-defined 

 with these great powers, while the stars preserved that lustre and distinctness 

 which from many thousand observations I knew they would retain. The sequel 

 has shown that my surmises were well founded, this proving to be the comet we 

 have lately observed. 



Mr. H. reduced all his observations on this comet to 3 tables. The first con- 

 tains the measures of the gradual increase of the comet's diameter. The mi- 

 crometers he used, when every circumstance is favourable, will measure ex- 

 tremely small angles, such as do not exceed a few seconds, true to 6, 8, or 10 

 thirds at most ; and in the worst situations true to 20 or 30 thirds : he therefore 

 gave the measures of the comet's diameter in seconds and thirds. The first 

 table, containing the measures of the comet's diameter, shows that, from March 

 17 till April 18, the apparent diameter increased from 1" 53'" to 5" 20'". 



The 2d table contains the comet's distances from several telescopic fixed stars, 

 from March 1 3 till April 1 g, and those distances expressed in minutes, seconds, 

 and thirds. And the 3d table contains the comet's angle of position with regard 

 to the parallel of declination of the same stars measured by a micrometer ; by 

 which means its places and apparent path might be determined. 



Description of a Micrometer for taking the Angle of Position. By Mr. JVm. 



Herschel,' of Bath. p. 500. 



Fig. 7, pi. 3, represents the micrometer inclosed in a turned case of wood, as 

 it is put together, ready to be used with the telescope, a is a little box which 

 holds the eye-glass, b is the piece which covers the inside work, and the box a 

 is screwed into it. c is the body of the micrometer containing the brass work, 

 showing the index plate a projecting at one side, where the case is cut away to 

 receive it. d is a piece, having a screw b at the bottom, by means of which the 

 micrometer is fastened to the telescope. To the piece c is given a circular mo- 

 tion, in the manner the horizontal motion is generally given to Gregorian re- 

 flectors, by the lower part going through the piece d, where it is held by the 

 screw e, which keeps the two pieces c and d together, but leaves them at liberty 

 to turn on each other. 



Fig. 8, is a section of the case containing the brass work, where may be ob- 

 served the piece b hollowed out to receive the box a, which consists of 2 parts 

 inclosing the eye lens. This figure also shows how the piece c passes through d, 

 and is held by the ring e : the brass work, consisting of a hollow cylinder, a 

 wheel and pinion, and index plate, is there represented in its place, f is the 

 body of the brass work, being a hollow cylinder with a broad rim c at the upper 

 end ; this rim is partly turned away to make a bed for the- wheel d. The pinion 

 e turns the wheel d, and carries the index plate a. One of its pivots moves in 



x 2 



