c ssx a 
XIII. On the oblique Refraction of Iceland Crystal. By William 
Hyde Wollaston, M. D. F. R. S. 
Read June 24, 1802. 
In the preceding communication, I have inserted two different 
measures of refractive powers, distinctly observable in the Ice- 
land crystal, as well as an estimate of its dispersive power ; but 
have reserved for a separate treatise, some remarks which the 
same mode of investigation has enabled me to make on its 
oblique refraction. 
The optical properties of this body have been so amply de- 
scribed by Huygens, in his Traite de la Lumiere y that it could 
answer little purpose to attempt to make any addition to those 
which he has enumerated. But, as the law to which he has 
reduced the oblique refractions occasioned by it, could not be 
verified by former methods of measurement, without considerable 
difficulty, it may be worth while to offer a new and easy proof 
of the justness of his conclusions. For, since the theory by 
which he was guided in his inquiries, affords (as has lately been 
shown by Dr. Young *) a simple explanation of several phe- 
nomena not yet accounted for by any other hypothesis, it must 
be admitted that it is entitled to a higher degree of consideration 
than it has in general received. 
According to that hypothesis, light proceeding from any 
luminous centre, is propagated by vibrations of a medium highly 
* Bake rian Lecture. Phil. Trans, for 3801. 
3 d 
MDCCCII. 
