sict. v.] DISSERTATION SECOND. 197 



an accurate expression of the phenomena of double refrac- 

 tion, cannot be doubted. When one enunciates the hypo- 

 thesis of the spheroidal undulations, he, in fact, expresses 

 in a single sentence all the phenomena of double refraction. 

 The hypothesis is therefore the means of representing 

 these phenomena, and the laws which they obey, to the 

 imagination or the understanding, and there is, perhaps, 

 no theory in opticks, and but very few in natural philoso- 

 phy, of which more can be said. Theory, therefore, in 

 this instance, is merely to be regarded as the expression 

 of a general law, and in that light, I think, it is considered 

 by La Place. 



To carry the theory of Huygens farther, and to render 

 it quite satisfactory, a reason ought to be assigned, why the 

 undulations of the luminous fluid are spheroidal in the case 

 of crystals, and spherical in all other cases. This would 

 be to render the generalization more complete ; and till 

 that is done, and a connexion clearly established between 

 the structure of crystallized bodies, and the property of 

 double refraction, the theory will remain imperfect. The 

 attention which at present is given to this most singular 

 and interesting branch of opticks, and the great number of 

 new phenomena observed and classed under the head of 

 the Polarisation of Light, make it almost certain that this 

 object will be either speedily accomplished, or that science 

 has here reached one of the immoveable barriers by which 

 the circle of human knowledge is to be for ever circum 

 scribed. « 



END OF PART SECOND 



25 



