19S OLservaiions upon the different Hypotheses 



explain the unusual refraction of the Iceland crystal by pression 

 or motion propagated, has not been hitherto attempted, to my 

 knowledge, except by Huygens, who for that end supposed two 

 several vibrating mediums within that crystal. But when he 

 tried the refraction in two successive pieces of that crystal, and 

 found them such as is .-ibove mentioned, lie coniessed himself 

 at a loss for explaining them." In his 2.Tth query, he also 

 states, " Are there not other original properties of the rays of 

 light, besides those already described ? An instance of another 

 original property is the refraction of Iceland crystal, described 

 first by Erasmus Bartoline, and afterwards more exactly by 

 Huygens." Sir Isaac then gives his own opinion, which, being 

 well known, need not be repeated. Indeed, with respect to the 

 Iceland crystal, Huygens was himself by no means satisfied with 

 his own views upon this subject : he observes in his Tractus 

 de Lnmine, page 67, when treating upon its double refraction, 

 " Quo arteni pacto id fiat, niliil reperire potui, quod mihi satis- 

 faceret." Also Haiiy in his Natural Piiilosophy gives the pre- 

 ference to Newton's explanation. He states*: " The theory of 

 Huygens," alluding to his theory of double refraction, " which 

 in general agrees with this mode of viewing the matter, is con- 

 nected in another way with an hypothesis not at all natural, re- 

 specting the elliptical figure of the waves of light, to which this 

 great roan attributes the refraction of aberration ; and he him- 

 self acknowledges his difficulty to reconcile with this hypothesis 

 the result of observations made with two rhomboids, in which 

 the rays sometimes merely change their functions, and some- 

 times are subdivided anew, in passing from one rhomboid into 

 another." — " \Vc are much more disposed to range ourselves here 

 again on the side of Newton, when we consider the extreme sim- 

 plicity of the law admitted by ttiis celebrated geometrician for 

 the refraction of the ray aberration." In short, so many diffi- 

 culties present themselves with respect to the undulating doc- 

 trine, that I am inclined to think it ought to be entirely aban- 

 doned. 



Havir.g thus given a rapid view of the objections to the Car- 

 tesian doctrine and its modifications, let us turn our attention to 

 that pro])osed by Sir Isaac Newton, which is, in fact, a modifi- 

 cation of the Pythagorean. Ey tliis philosopher light is sup- 

 posed to consist of particles emitted from the luminous body in 

 every direction. The first objection is, that as the ravs of light 

 pass from every par^; of a luminous 'oody, they must interfere 

 and destroy each other ; or that i,erception must become con- 

 fused, and objects be represented indistinctly. Canton has removed 



* Dr. Gregory's Trans, of Ilaiij's Nut. Fliil. 



this. 



