Refraction of Iceland Crystal . 383 
emergent ray becomes parallel to the surface, the ordinate of 
refraction is then a semidiameter of the spheroid ; and, accord- 
ingly, the refractive power of this spar, when examined by 
means of a prism in different directions, should be found to vary 
as that semidiameter which coincides with the plane of inci- 
dence and refracting surface. 
The observations that I have made on this substance, accord 
throughout with this hypothesis of Huygens ; the measures 
that I have taken, correspond more nearly than could well hap- 
pen to a false theory, and are the more to be depended on, as 
all my experiments, excepting the last, were made prior to my 
acquaintance with the theory, and their agreement was deduced 
by subsequent computation. 
Exp. 1. The oblique refraction of this spar is rendered visible, 
by cementing a surface of it to a prism of flint-glass, with a 
little balsam of Tolu. When the line of sight bisects an acute 
angle of a natural surface of the spar, the refractive power is 
seen to be less than in any other direction, and may be expressed 
by the sine 1,488, or its reciprocal 0,67204. 
Exp. 2. When the plane of incidence is parallel to one of 
the sides, the power is 1,518, of which the reciprocal is 0,6587. 
Exp. 3. Iiya direction at right angles with either side, it is 
found still higher, being 1,537, or * ts reciprocal 0,6506. 
Exp . 4. And, in the plane bisecting an obtuse angle, the re- 
fractive power of the natural surface appears greatest, and is 
expressed by the sine 1,571, or its reciprocal 0,6365. 
Exp. 5. When either of the two greatest solid angles of the 
spar contained under three obtuse angles, is cut off by a po- 
lished surface making equal angles with each of its sides, the 
same refractive power 1,488 is found in all directions. By the 
3D 2 
