Description of a reflective Qoniomeicr, 97 



cision, to be 104° 28' 40'' : a result deduced from the sup- 

 posed posuion Oi its axis at an angle of 43^ with each of the 

 surfaces, and from other seducing circumstances of appa* 

 rent harmony hv simple ratios. But however strong the 

 presumption might he that this angle, which by measure- 

 ment aj)proaches to45*', is actually so, it must nevertheless 

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

 surfaces to each other is very nearly, if not accurately, 103^, 

 as it was formerly determined to be byHuygens*; and 

 since the measure of the superficial angle given by Sir Isaac 

 JsTcwton t corresponds with this determination of Huygcns, 

 his evidence may be considered as a further confirmation of 

 Ihc same result ; for it may be presumed, that he would not 

 adopt ih& measures of others, without a carefql examina- 

 tion. 



IN THE ANNEXED PLATE, 



ab. Is the principal circle of the goniometer graduated 

 on its cdgQ. 



c c. The axle of the circle, 



d. A milled head by which the 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 

 SLS a vernier to every five minutes. 



/z. The extremity of a small spring, by which the circle 

 is stopped at 180**, without the trouble of reading off. 



a and k k. Are two centres of motion, the one horizon* 

 tal, the other vertical for adjusting the position of a crystal : 

 one turned by the handle /, the other by the milled head m* 



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

 (in the centre of all the motions), with one of its surfaces 

 as nearly parallel as may be to the milled head w, is next 

 rendered truly parallel to the axis by turning the handle I 

 till the reflected image of a horizontal line is seen to be ho- 

 rizontal. 



By means of the milled heady", the second surface is then 

 brought into the position of the first; and if the reflected 

 image from this surface is found not to be horizontal, it is 

 rendered so by turning the milled head tn ; and since this 

 motion is parallel to the fir^t surface, it does not derange 

 the preceding adjustment. 



* Huygenii Opera Rdi(jua, torn. i. p. 73. — Tract, de Limine. 



f Newton'i Optics, 8vo. p. 329. Qu, 25, concerning Iceland Crystal, 



Vol.35. No. 142. Feb. 1810, G XVI. O** 



