379 



serves, that, supposing the function v to be known, ^' we 

 can immediately apply the general method given in the Me- 

 canique Analytique, and which appears to be more especially 

 applicable to problems that relate to the motions of systems 

 composed of an immense number of particles mutually acting 

 upon each other." Such is certainly the great advantage of 

 starting with that general principle ; but the chief difficulty 

 attending it, namely, the determination of the function v, on 

 which the success of the investigation essentially depends, has 

 not been surmounted by Mr. Green, who has consequently 

 been led to very erroneous results, even in the simple case 

 of uncrysialUzed media, to which his researches are exclu- 

 sively confined. In this case Mr. Mac Cullagh's theory 

 confirms the well-known formulae of Fresnel, one of which 

 Mr. Green conceives to be inaccurate, and proposes to 

 replace by a result of his own, which, however, will not 

 bear to be tested numerically. The present theory applies 

 with equal facility to all media, whether crystallized or not, 

 and is distinguished throughout by the singular elegance 

 and simplicity of its analytical details ; a circumstance which 

 the author regards as a strong indication of its truth. 



Mr. Lloyd exhibited to the meeting a specimen of a re- 

 markable substance recently found in the principality of 

 Carolath, in Silesia. It formed part of a cloth of 200 square 

 feet in surface now in the possession of the King of Prussia. 

 No description of this substance has yet been published ; but 

 Major Sabine and Mr. Lloyd were informed by Baron Hum- 

 boldt (by whom the present specimen was kindly given) that 

 M. Ehrenberg had examined it microscopically, and had 

 found it to be an organic substance, consisting partly of ve- 

 getable and partly of animal matter ; — the vegetable compo- 

 nent being the conferva rivularis, the animal different species 

 of Infusoria, of the family known by the name of Bacillaria. 



To illustrate the origin of this substance, Mr. Lloyd. 



