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



lamp, illuminating the circular lines, cannot come to the eye ; 

 and that the field of view remains dark. In the eye-glasses, 

 which I made for the lamp-circular micrometers, this has been 

 very well accomplished. In order to keep off' the light coming 

 from the lamp as much as possible from the eye, a great deal 

 will depend also upon the form of the setting of the eye-glass. 



The micrometer with which the astronomer M. Soldner 

 made some observations, and who under favourable circum- 

 stances will make more with it, contains circular lines, which 

 follow one after the other, as is represented on a larger scale 

 in Fig. 1. The smallest circular line appears to the naked 

 eye as a small dot, and is only distinguished by the eye-glass. 

 With a higher power, which in a telescope of 60 inches focus 

 magnifies 110 times, 5 circular lines are seen, including the 

 smallest : through the middle eye-piece, magnifying 62 times, 

 8 circular lines are seen ; and with the lowest eye-piece, mag- 

 nifying 45 times, eleven circular lines are seen. As with many 

 circular lines it could perhaps not be correctly judged at the 

 moment how many of the lines the star has cut, and an error 

 might easily take place, I gave to the 5th and 6th, the 8th 

 and the 9th larger distances from each other, than to the rest ; 

 so that it is known in a moment with which circular line the 

 observer is occupied. With the magnifying power of 45 

 times, the largest circular lines are illuminated somewhat 

 weaker than the rest, and are less plainly seen ; for which rea- 

 son the weaker one is used only in great differences of decli- 

 nation, where the stronger oculars cannot be made use of; 

 and also for this reason, that with a less magnifying power 

 the same exactness is not possible as with higher ones. 



The different circular lines of the above-mentioned micro- 

 meter have the following dimensions, the Paris inch taken as 

 unity: 



These dimensions I measured with the microscope, with 

 which I determined the breadth of the interstices and thick- 

 ness of the threads of those squares, which appertain to the phe- 

 nomena 



