i\I. Frauenhofer's Description of a new Micrometer. 211 



phram of the telescope, the ingress of a star into the field of 

 view, cannot be observed with the same exactness, as its egress ; 

 because, since we do not know at which place it will enter, the 

 eye is not directed to that point, and is consequently only di- 

 rected towards it when the star is already in the field of view. 

 With the open diaphram it is likewise difficult to let the star 

 describe any given chord of the field ; which is in fact neces- 

 sary, because small segments give the declination, but larger 

 ones the right ascension, more correctly. Instead of the open 

 diaphram they have also frequently made use of a small ring, 

 which, by means of four bars, is suspended in the field of 

 view. A small ring can only accidentally succeed in being per- 

 fectly round, as in good observations it ought to be ; and even 

 from its being suspended by wires, which may be stretched, 

 its perfect form may be lost ; and even the expansion or con- 

 traction of the metal with which it is connected, may alter it. 



In order to have in the field of view of a telescope a small 

 unchangeable ring, whose inner edge is exactly round, and 

 remains so under all circumstances, I cut a round hole in a 

 thin plate of glass, and fixed a small steel ring into it, in the 

 same manner as glass-lenses are fixed in brass mountings ; 

 viz. by laying over the projecting margin by means of a bur- 

 nisher. After this ring was fixed in the plate glass the inner 

 edge could be turned in a lathe exactly circular, which is done 

 in a manner that leaves no doubt respecting the necessary ac- 

 curacy. As the objects are seen through the glass as well as 

 without it, we see the star approach the external edge of the 

 small ring, and know where it is to appear at the inner edge ; 

 the observer is consequently prepared for the moment, and 

 the ingress at the inner edge can be observed with the snme 

 exactness as the egress from it. Partly for the purpose of ob- 

 taining, at the transit of both stars, the exact difference of de- 

 clination, and that of the right ascension, partly to enable me 

 to use one and the same micrometer likewise for smaller dif- 

 ferences of declination, I have introduced two small steel 

 rings, in the above-mentioned manner, into the field of view, 

 where the diameter of the one is considerably larger than that 

 of the other, and both of which are plainly seen at the same 

 position of the eye-glass. 



Although the observations with this circular micrometer are 

 more to be depended on than with the usual one, yet even 

 these leave much more to be wished, particularly in the ob- 

 servation of comets; and more particularly since the differ- 

 ences of declination, derived from different observations, fre- 

 quently deviate considerably from each other. One of the 

 causes is, that in a comet, which is always badly defined, the 



D d 2 centre 



