Notices respecting Neiv Books. 61 



tious are incident in the same phase on a glass-reflector at the 

 angle of complete polarization, no reflexion takes place if the 

 plane of reflexion be perpendicular to that of the polarization of the 

 original ray. As the reflexion is considered to bring the two rays to 

 the same plane of polarization, it is imagined that the non-reflexion 

 is caused by interference, and, as the lengths of the respective 

 paths do not accord with this supposition, it is further supposed 

 that one of the rays has suff'ered the loss of half an undulation. 

 The necessity for these gratuitous suppositions arises out of the 

 inadequacy of the vibratory theory of light to explain the phse- 

 nomena. Accordmg to the hydrodynamical theory of polariza- 

 tion, which I have given in vol. viii. part 3 of the ' Cambridge 

 Philosophical Transactions,' the two parts of a plane-polarized 

 ray, bifurcated in the manner above stated, constitute, on meeting 

 in the same phase, a plane-polarized ray equivalent to the origi- 

 nal one, and having the same plane of polarization. It is for 

 this reason, solely, that the compound ray is not reflected when in- 

 cident on the glass- reflector in a plane perpendicular to that of 

 original polarization. This simple explanation essentially de- 

 pends on an investigation of the properties of polarized i-ays by 

 means of partial differential equations, as will be seen by con- 

 sulting the paper above cited. 



These are the only instances of the supposed loss of half an 

 undulation in physical optics which i-equire to be brought under 

 discussion. 



Cambridge Observator)', 

 June 20, 1869. 



XII. Notices I'esjiccting New Books. 



Etudes sur la Metamorphisme des Roches. Par M. Delesse, Ingenieur 

 des Mines, Professeur de Geologic a I'Ecole Normale. 8vo. Dal- 

 mond et Dunod, Paris. 



TN this work M. Delesse has brought together part of his re- 

 -*- searches into the interesting subject of the metamorphism of 

 rocks, the publication of which had hitherto been scattered through 

 the Annates des Mines. 



He first of all considers metamorphism under two heads, namely, 

 normal or general metamorphism, which proceeds from causes which 

 are not obvious, and is produced on a large scale, and abnormal or 

 special metamorphism, or what is called contact metamorphism. It is 

 with the latter that he alone concerns himself in this work, in which 

 he gives a great number of details on the metamorphic action pro- 

 duced by eruptive (intrusive) rocks. This action is twofold, since 

 not only does the intrusive rock exert a metamorphic action on the 

 rock which it penetrates, but it is itself often more or less changed 

 by the rock with which it comes in contact. 



