21 '2 M. Frauenhofer's Description of a new Micrometer. 



centre must be estimated ; but we cannot judge with exactness 

 whether the half of it has made its ingress or egress, because 

 the other half is not seen, and the mere approach of a weakly 

 illuminated object to a proportionally broad dark ring causes 

 easily a deception in estimating the correct time. Through 

 the eye-glass, which cannot well be achromatic, the inner edge 

 of the small steel ring becomes blue, but the outer one is seen 

 to be red ; the star on the contrary is blue outwards and red 

 inwards, so that, for instance, at the egress of the star from 

 the inner circle of view, the blue end of it gets to the blue 

 edge of the small steel ring, and the star lengthens itself still 

 more, which must cause an uncertainty respecting the time of 

 its egress. As the star always lengthens itself in the direction 

 of the centre of the field of view, and both the passing stars 

 usually cut different segments of the circle, the errors of ob- 

 servation cannot easily compensate themselves. In an eye- 

 piece with compound glasses the colours of the star can indeed 

 be lessened ; but the small ring would be seen the more co- 

 loured. For other reasons, too, these eye-glasses would not be 

 advisable. 



The reason why the observations in a transit instrument are 

 capable of so great a degree of exactness, is, without doubt, 

 because the threads are so thin, that they hardly cover the 

 star, or do so for a moment only, and that the space passed 

 within a second of time next to the thread can be divided. 

 If the observations with a circular micrometer were to give a 

 similar exactness, then the circles in the field of view ought to 

 be as thin as those threads, but would require to be illuminated 

 in the dark field of view, because in comets the field cannot be 

 illuminated. 



I tried to cut with a diamond fine circular lines upon a thin 

 piece of plate glass ; and having brought it into the focus of 

 the telescope, illuminated the cut lines in the same manner as 

 I had formerly done in lamp-wire micrometers. But in what- 

 ever way I altered the illumination, there were always small 

 segments only of the cut lines weakly illuminated ; viz. those 

 segments upon which the light which came from the lamp fell 

 nearly vertically. It is also the case with the wire-lamp mi- 

 crometer, that the threads are only then illuminated strongest, 

 when the light falls vertically upon them ; thus, for instance, 

 the vertical wire can be splendidly illuminated, while the ho- 

 rizontal one is quite unilluminated. By increasing the lamps, 

 which were used for illuminating the cut lines, the object 

 could not be accomplished. I observed, however, that already, 

 with one lamp, the small particles, accidentally adhering to 

 the glass, were illuminated splendidly bright, and had the ap- 

 pearance 



