Notes on some Points in the Theory of Light. 213 



same principles, and leading, by the same relations among con- 

 stants, to formulas identical in every respect with, his earlier 

 results, was advanced independently, and nearly at the same 

 time, by M. Neumann of Konigsberg.* A coincidence so re- 

 markable would be looked upon, not unreasonably, as a strong 

 argument in favour of the theory ; though it must be allowed 

 that, in the effort to extend fhe knowledge of any subject, 

 there is a tendency in different minds to adopt the same errors 

 respecting 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,f 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 strahlende 

 Wdrme, oder chemisch wirkend, oder als irgend ein anderes Agens"~\ 

 and he thinks that the nature of this ray will be more easily 

 investigated, if the laws of reflexion 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 dis- 

 coveries which have been made of late years respecting the po- 

 larization 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 dif- 

 ferent from those of the supposed third ray, which are normal, 

 or nearly so. There is every reason to believe that the vibrations 

 of the chemical rays are also transversal ; and we may confi- 

 dently assert, that the three species of rays those of light and 

 heat, and the chemical rays are produced not only by vibra- 

 tions of the same medium, but by the same kind of vibrations, 

 propagated with nearly the same velocities. If, therefore, the 

 third ray of MM. Cauchy and Neumann has any existence, it 

 must be referred to " some other agent," the nature of which it 

 is impossible to conjecture. 



Enough has now been said to show that the optical theory 



* Poggendorff's Annals, Vol. xxv. p. 418. 

 f Ibid. p. 454. 



