M. Frauenhofer's Description of a new Micrometer. 213 



pearance of stars in the field of view. If therefore the circular 

 lines were composed of small dots, then they would be illumi- 

 nated by one lamp equally at all places. We have however 

 to contend with great difficulties, to make a circle consisting 

 of dots exactly round. 



I recollected that a line deeply etched in glass by fluoric 

 acid gas, examined under the microscope, consists in its depth 

 of inequalities, and has nearly the same appearance as if it 

 consisted of dots. The circular lines deeply etched in this 

 manner were illuminated sufficiently and pretty nearly equal 

 at all parts, by a lamp. The glass on which lines are to be 

 etched is covered for this purpose with a very thin coat of 

 etching ground, on which the lines, intended to be etched, are 

 to be scratched with a steel point. Covering the glass with 

 leaf-gold, instead of etching ground, as is frequently done for 

 etching with gas, is not advantageous for circular micrometers, 

 because the fluoric acid gas acts, next to the scratched line, 

 also beneath the gold, and in etching deep, the polish of the 

 glass likewise suffers in other places. With a brittle etching 

 ground the scratched lines become impure, and the gas, in 

 etching the lines deep, acts next to them beneath the etching 

 ground. With a too soft etching ground the etched lines 

 easily receive unequal strength. The etching ground must 

 intimately cohere with the surface of the glass, and be of such 

 a consistency, that the steel point, in scratching, cuts only fine 

 threads, which repeated practice will teach. If the fluoric acid 



f'as acts too short a time on the glass, then the etched circular 

 ines are illuminated but very weakly by the lamp ; too strong 

 etching, even with good etching ground, makes the lines 

 coarse and rough. Common plate glass is unequally acted 

 upon at different places by the fluoric acid gas. The glass 

 used therefore for lamp-circular micrometers must be very ho-, 

 mogeneous. In several sorts of glass, the time which the a<?id 

 takes to act, differs, and some sorts of glass, deep even as they 

 may be etched, never give a circular line, which shall appear 

 in all its parts strong and equally illuminated by the lamp. 



In order to give the circles made by the scratched lines the 

 degree of exactness that is required, and to scratch upon the 

 same glass many circular lines, which are exactly concentric, 

 I contrived a particular engine, whose description is here su- 

 perfluous ; with it circular lines of *003 to 1*3 inch diameter 

 can be made at any given distance from each other. 



We have to contend with some difficulties, in finding a con- 

 struction of eye-glasses, which at one and the same position 

 plainly show the inner and outer circular lines in the field of 

 view, and which are so contrived, that the light from the 



lamp. 



