incT systems of molecules, an hypothesis which M. Cauchy has 

 already considered with his usual generality, but without 

 making any precise application of it. {Exercices d^ Analyse 

 et de Phyniqne Mnthemntiqve, Tom. i. p. 33.) 



Perhaps one cause why M. Cauchy's views on the subject 

 of double refraction have met with such general acceptance, 

 may be found in the fact, that a theory setting out from the 

 same principles, and leading, by the same relations among 

 constants, to formulas identical in every respect with his 

 earlier results, was advanced independently, and nearly at the 

 same time, by M. Neumann of Konigsberg {Pog<Tendorff"s 

 Annals, vol. xxv. p. 418). A coincidence so remarkable 

 would be looked upon, not unreasonably, as a strong argu- 

 ment in favour of the theory ; though it must be allowed that, 

 in the cfFort to extend the knowledge of any subject, there is 

 a tendency in different minds to adopt the same errors re- 

 specting it, as well as the same truths ; a fact of which we 

 have seen other examples in the course of the present 

 article. 



According to M. Neumann (ibid. p. 454), the " third 

 ray," not being perceived as light, must manifest its existence 

 as radiant heat, or as a chemical power, or as some other 

 agent [" als slrahlende JVdrme, oder chemisch wirkevd, oder 

 als irgend ein anderes Agens"], and he thinks that the nature 

 of this ray will be more easily investigated, if the laws of re- 

 flexion shall be deduced from the aforesaid theory. But we 

 have seen that the laws of reflexion are, to all appearance, at 

 variance with the theory, and they take no account whatever 

 of the third ray. Besides, the discoveries which have been 

 made of late years respecting the polarization of radiant heat, 

 and the strong analogies that have been traced between it 

 and light, amount to a demonstration that its vibrations are 

 transversal, and of course essentially different from those 

 of tlie supposed third ray, which are normal, or nearly so. 

 There is every rea.son to believe that the vibrations of the 



