reflective Goniometer | 257 
to be by Huygens ;* and since the measure of the superficial 
angle given by Sir Isaac Newton^ corresponds witli this 
determination of Huygens, his evidence may be considered 
as a further confirmation of the same result ; for it may be 
presumed, that he would not adopt the measures of others, 
without a careful examination. 
in the annexed plate, 
a b. Is the principal circle of the goniometer graduated on 
its edge. 
cc. The axle of the circle. 
d. A milled head by which the circle is turned. 
ee. The small axle for turning the crystal, without moving 
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 the circle is 
stopped at 180°, without the trouble of reading off. 
ii and kk. Are two centers of motion, the one horizontal, 
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 center of all the motions), with one of its surfaces as 
* Huygenii Opera Reliqua, Tom. I. p. 73 — Tract, de Lumine. 
+ Newton’s Optics, 8vo. p. 329. Qu. 25, concerning Iceland Crystal. 
