38a 
Dr. Wollaston on the oblique 
elastic, that pervades all space. In ordinary cases, the incipient 
undulations are of a spherical form ; but, in the Iceland crystal, 
light appeared to Huygens to proceed as if the undulations 
were portions of an oblate spheroid, of which the axis is parallel 
to the short diagonal of an equilateral piece of the crystal, and 
its centre the point of incidence of the ray. 
From this spheroidical form of the undulations, he deduces 
the obliquity of refraction ; and lays down a law, observable 
in all refractions, at any surface of the spar, whether natural 
or artificial, which bears the closest analogy to that which ob- 
tains universally at other refracting surfaces; for as, in other 
cases, the ratio is given between the sine of incidence and sine 
of refraction, (or ordinate of the spherical undulation propagated,] 
so, in the Iceland crystal, the ratio between the sine of incidence 
and ordinate of refraction (in any one section of the spheroidical 
undulation) is a given ratio. 
If ABD (Fig. 1, Plate XV.) be any surface of the spar, let 
FHOK be a section of the spheroid through its centre C, and 
RC any ray of light falling on that surface; draw FO a dia- 
meter of the spheroid, in the plane of incidence RVO, and CT, 
its semiconjugate diameter, in the plane of refraction FTO. 
Then, if Cl be the refracted ray, VR, the sine of incidence, shall 
be to El, the ordinate of refraction parallel to FC, in the con- 
stant ratio of a given line N to the semidiameter FC. 
In any other plane of incidence, the ratio of sine to ordi- 
nate is also constant ; but it is a different ratio, according to the 
magnitude of that diameter in which the plane of incidence 
intersects the ellipse FHOK. 
When the incidence of a ray passing from any medium of 
greater density upon a surface of this spar, is such that the 
