REFLECTIVE GONIOMETER. igS 



to be employed as a test of the correctness of such a oonio- 

 meter, by Jinv one who is not convinced of its accuracy 

 from a distinct conception of the principles of its construc- 

 tion. 



The inclination of the surfaces of a primitive crystal of Mistake in the 



carbonate of lime is stated, with i^rredt ai^pearance of preci- f"^'^^ "f,^*^"^' 



, 1 , , P , bonate of lim? 



tiion, to be 104 28 40 : a result deduced irom the sup- aorrectedby it, 



posed position of its axis at an aui^le of 45° with each of the 

 suriaces, and from other seducing- circumstances of appa- 

 rent harmony by simple ratios. ]3ut however strong the 

 presumption might be that this angle, which by measure- 

 ment approaches to 45°, is actually so, it must nevertheless 

 be in fact about 45° 20' ; for I tind the inclination of the 

 surfaces to each other is very nearly, if not accurately 105°, ^nt' made to 

 as it was formerly determined to be by Huj'^geas*; and n,cas;in>s of 

 since the measure of the superficial angle given by Sir Huygens and 

 Isaac Newton "f corresponds with this determination of 

 Huygens, his evidence may be consiciered as a further con- 

 firmation of the same result ; for it may be presumed, that 

 he would not adopt the measures of olhcrs, without a care- 

 ful examination. 



In the annexed plate, 



a b. Is the principal circle of the goniometer graduated Explanation 

 on its edge. of the plate. 



cc. The axle of the circle. / 



d. A milled head by v4iich tlie circle is turned. 



ee. The small axle for turning the crystal, without mov- 

 ing the circle. 



f. A milled head on the small axle. 



g. A brass plate supported by the pillar, and graduated 

 as a vernier to every five minutes. 



h» The extremity of a small spring, by which Ihe circle is 

 stopped at 180°, without the trouble of reading oil'. 



a and kk. Are two centres of motion, the one horizontal, 

 the other vertical, for adjusting the position of a crvstal : one 

 turned by the ])andle /, the other by the milled head m. 



The crystal being attached to a screw-head at the point n 



* Iluygenii Opera Reliqua, Tom. I, p. 73 — Tract, de Lnmiue. 



f Nev7lou's 0])tics, 8yo, p. G29, Qii. x.',=i, roncemiiig icthmd Cr)^st;'l. 



0'2 (itt 



