DESCRIPTION C¥ THi: C. K APHO M LT£R. IS3 



etch other. It is known that this angle is equal to that of two 

 lines drawn to any Tingle point of the edge which unites thcfe 

 planes, provided they be perpendicular with this edge, and 

 fituated in the planes themfelves. To find this angle, we dif- 

 pofe the in ft rumen t in fuch a manner, that the portions c Q 

 and c B of tiie two indexes mall have no fpace between themr 

 felves and the points in queftion, and that their fides mall be 

 perpendicular to the edge at which they join. In this caie, 

 the faces which embrace the cryilal are tangents to the two 

 planes whofe incidence we with to meafure. This being 

 done, we look on the circumference of the inflrument for the 

 degree marked by the fiducial edge a* z, or the angle which 

 this line forms with that which pafles through the center c 

 and the point zero, which angle is equal to that formed by 

 the two portions G c and c B of the indexes, becaufe verti- 

 cally oppofite. k 



It is an advantage to be able to fhorten thefe parts at plea- Advantag* of 



fure, in order to avoid the obflacles which would otherwife fl IQ r ten,n & tnc 



legs, 

 render the operation im practicable, either from the gangue to 



which the cryilal adheres, or from the contiguous cryflals by 



which it is partly furrounded. 



Cafes, however, occur in which this expedient would not In fome cryftals 



be fufficient, and we mould find ourfelves embarrafled by the this «P e ? cnt is 



... J not lumcient} 



part of the femicircle fituated towards M, it' its pofition were 



invariable. The ingenious inventor of the inflrument has 



guarded againfl this inconvenience by means of the following 



mechanifm : 



The fcrew fituated at c holds not only the two indexes, but but part of the 



alfo a ft eel radius placed below the brafs arm, upon which the Clrc " ,ar limb 

 . , ~ „ . . ,. , „ ,. , r^, . „muftbcocca- 



mdex Gr is immediately applied. I he upper extremity of fionally re- 



this radius, or that which is fituated towards O, has an exca* »noved. 

 vation into which a brafs flem enters, which is likewife pro- 

 vided with a fcrew. The femicircle is betides divided into 

 two pieces, at the place of the 90th degree ; fo that, by means 

 of a hinge at that part, the quadrant T M may be folded be- 

 hind the quadrant T N, and is, as it were, fupprefTed. 

 When we with to perform this movement, we loofen the fcrew 

 which fupported the upper part of the radius c O, difengage 

 the excavation which terminates this radius from the fcrew 

 which was inferted into it, and draw back the radius till it 

 lies beneath the brafs arm which fupports the index G F. 



When 



