'450 On Crystallography . 



this angle is equal to th^t of two lines drawn from one 

 and the same point of ihe edge which joins these planes, 

 with the condition that they are perpendicular to this ridge 

 and laid down on the same planes. In order to have this 

 ^gle, we shall arrange the instrument so that the portions 

 c G, c B of the two arms may leave no lii^ht between them 

 and the planes in question, and at the same time iheir edges 

 ma) be perpendicular to the edge of junction. In this case, 

 the faces wh.ch embrace the crystal are tangents to the two 

 planes whose incidence we seek for. This being done, we 

 shall seek on the circumference of the instrument, the 

 degree which the index hue sz marks, or the angle 

 which this line forms with that, which passes by the centre 

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

 formed by the two portions' G c, cB of the arms, since it 

 is opposite to it at the summit. 



It is an advantage to be able to shorten these parts at 

 pleasure, to avoid the obstacles which would render the 

 operation impracticable, and which might be occasioned 

 either by the matrix to which the crystal adheres, or from 

 the adjoining crystals in which it is partly fastened. 



But there are cases in which this precaution is not suf- 

 ficient, and in which we should find ourselves constrained 

 by the part of the semicjrcle situated towards M, if its 

 position was invariable. The ingenious inventor, of the 

 instrument has guarded against this inconvenience by the 

 following contrivance. 



. The stalk at c has, besides the two arms, a stay or rod 

 of steel placed below the copper rule on which the arm 

 GF is immediately applied. The upper extremity of this 

 rod, or that which is situated towards O, has a hole into 

 which a steel peg also enters, furnished with a screw in a 

 similar manner. In addition to ttiis the semicircle is di- 

 vided at 90% so that, by means of a hinge with which it is 

 provided at the same place, the quarter of the circle TM 

 is folded below the quarter of the circle T N, and is as it 

 were suppressed. When we wish to e;(ecute this move- 

 ment, we must loosen the screw which fastened the upper 



'^^art of the rod cO, we must disengage the hole at the end 

 pf this rod from the screw which is inserted into it, and 



•we must pull down the rod until it is beneath the copper 

 rule ^.hich has the arm G F. When the angle measured 

 exceeds (>0^, we must return the quarter circle TM to its* 

 place, in order to ascertain its value. 



It will be easy to appi-eciate the utility of the goniometer, 

 if we reflect how inteicsting it is that descriptions of cry- 

 stals 



