210 M. Frauenhofer's Description of a new Micrometer. 



paratus which we employ in all experiments on light; and 

 we must guard against being deceived by any peculiarities or 

 affections in our own organs of vision, which we need not fear, 

 provided only we make the same experiments repeatedly and 

 with due attention. 



[To be continued.] 



XXXV. Description of a tiew Micrometer. By M. Frauen- 

 hofer of Munich. * 



r I "'HERE is probably no instrument, if we except the helio- 

 ■*■ meter, better adapted for determining the right ascension 

 and declination of two stars, than the circular micrometer. To 

 this simple instrument, which can be applied to any telescope 

 provided with a good stand, we owe, besides many other im- 

 portant observations, a great part of those made on comets. 

 Improvements in this instrument are therefore desirable. 



Some astronomers still prefer the rhomboidal micrometer 

 for determining the relative places of two stars. It cannot be 

 denied that the calculation of observations made with such 

 a micrometer, is more simple than that made by the circular 

 micrometer, and that the result possesses about the same de- 

 gree of exactness, whether the difference of declination of both 

 stars be great or small; while in observations with the circular 

 micrometer we ought to have, for a smaller difference of de- 

 clination, a smaller circular micrometer also. Any one pos- 

 sessed of practical knowledge will soon perceive that a rhom- 

 boid, in whatever manner it may be made, cannot obtain the 

 prescribed form to such a degree of accuracy as is required 

 for good observations ; whilst, on the contrary, there are seve- 

 ral means of making a circle to a great degree of exactness. 

 It cannot be expected to file out a hole exactly in a rhom- 

 boidal shape. We may indeed obtain a rhomboid with straight 

 lines, by screwing together bars of equal breadth ground per- 

 fectly straight ; but to give the required angles of the rhom- 

 boid is not very practicable. In stretching four wires, in the 

 form of a rhomboid, we have to encounter the same difficulties, 

 as far as regards the angles of the rhomboid; besides this, 

 there is seldom a piece of wire exactly straight, even if ever so 

 strongly stretched ; but thin wires cannot be made use of for 

 the purpose, because in a dark field of view they cannot be 

 seen. This is the reason why circular micrometers have such 

 a decided preference. 



With the circu'ar micrometer, consisting merely of the dia- 



* From M. Schumacher's Astronomische Kachrichten, No. 43. 



phram 



