of a more complete Map of the Heavens. 175 



is moved until a brighter observed star passes the illuminated 

 micrometer ; and the difference is then read off. It is clear 

 that this method is to be applied to very small stars only, as 

 it requires more time than the direct observation on the illu- 

 minated bars. 



Although we have thus described the essential parts of the 

 construction and use of this micrometer, we hope that those 

 who are engaged in revising the maps of the heavens will still 

 be pleased to see a detailed description of its adaptation to a 

 Fraunhofer's sweeper ; that being the instrument intended by 

 Professor Bessel for the revision of the maps. In determining 

 the construction of such an instrument, we must take care, 



1°. That the small object-glass should intercept as little of 

 the light coming to the larger object-glass as possible ; and yet 

 should present a sufficiently clear image of the micrometer 

 plane. I have found by trial, that the limits of the measure 

 of brightness of the same should be W, 3 and 0'"'8 Paris lines : 

 2°. That the micrometer plane should be entirely covered 

 by the small object-glass in regard to the object-glass of the 

 eye-piece : 



3°. That the micrometer plane should extend over the 

 whole field of view. 



Besides these conditions, there are restrictions arising from 

 the mechanical execution of the small micrometer plane, the 

 intensity of the light for its illumination, &c. &c. ; so that 

 after the limits, between which the dimensions must be se- 

 lected, have been found by calculation, practice only will lead 

 to the most advantageous combination. In this manner I 

 found, if the dimensions of the telescope are as follow, — viz. 



Focal length of the object-glass 288 Paris lines. 



Aperture 34? do. 



Diameter of the object- {Collective-) glass of > ~ _, 



the eye-piece \ °* 



Field of view 4° 



Magnifying power 15 times, — 



that the following construction of the micrometer is the most 

 advantageous: viz. 



Focal length of the micrometer object-glass 74'" 



Aperture 9 



Diameter of the object-glass with frames 11*4 



External or greatest diameter of the micrometer plane 6' 5 



Interior or smallest 5*0 



One division of the micrometer plane 0*1 



Fig. 1. represents the micrometer as adapted and fitted to 

 the telescope. 



The micrometer plane (a) consists of a small plate of silver 



(a piece 



