NATURAL PHILOSOPHY. 211 



therefore, to be no means left to the theorist to decide between these conflict- 

 ing hypotheses, until Prof. Stokes recently, in applying the dynamical theory 

 of light to other classes of phenomena, found one in which the effects should 

 differ on the two assumptions. When the light is transmitted through a fine 

 grating, it is turned aside, or diffracted, according to laws which the wave- 

 theory has explained. Now, Prof. Stokes has shown that, when the incident 

 light is polarized, the plane of vibration of the diffracted ray must differ from 

 that of the incident, the two planes being connected by a very simple rela- 

 tion. It only remained, therefore, for observation to determine whether the 

 planes of polarization of the incident and refracted rays were similarly re- 

 lated or not. The experiment was undertaken by Prof. Stokes himself, and 

 he has inferred from it that the original hypothesis of Fresnel is the true one. 

 But, as an opposite result has been obtained by M. Holtzmann, on repeating 

 the experiment, the question must be regarded as still undetermined. 



The difference in the experimental results is ascribed by Prof. Stokes to 

 the difference in the nature of the gratings employed by himself and by the 

 German experimentalist, the substance of the diffracting body being sup- 

 posed to exert an effect upon the polarization of the light, which is diffracted 

 by it under a great obliquity. I learn from Prof. Stokes that he proposed to 

 resume the experimental inquiry, and to test this supposition by employing 

 gratings of various substances. If the conjecture should prove to be well 

 founded, it will greatly complicate the dynamical theory of light. In the 

 mean time the hypothesis is one of importance in itself, and deserves to be 

 verified or disproved by independent means. I would venture to suggest 

 that it may be effectively tested by means of the beautiful Interference-refrac- 

 tor of M. Jamin, which the inventor has already applied to study the effects 

 upon light produced by grazing a plate of eny soluble substance inclosed in 

 a fluid. It is well known that the refractive index of bodies increases with 

 their density ; and the theory of emission has even expressed the law of 

 their mutual dependence. That theory, it is true, is now completely over- 

 thrown by the decisive experimentum crucis of M.M. Fizeau and Foucault. 

 It was, therefore, probable, a priori, that this law the only one peculiar to 

 the theory should be found wanting. Its truth has recently been put to 

 an experimental test by M. Jamin. Water, it is known, has its maximum 

 of density at about 40 of Fahrenheit ; and accordingly, if Newton's law were 

 true, its refractive index should also have a maximum value at the same 

 tcinperatare. This has been disproved by M. Jamin, by observing the inter- 

 ference of two rays, one of which has passed through air, and the other 

 through water; and thus the last conclusion of the emission-theory has been 

 set aside. President's Address, British Association, 1857. 



EXPERIMENTS ON THE ACTINIC POTVTR OF THE SUN. 



The following account of experiments made by Mr. J. J. Waterston, at 

 Bombay, on the limit of the photographic power of the sun's direct light, 

 have recently been communicated to the Astronomical Society, G. B. They 

 were made with the view of obtaining data in an inquiry as to the possibility 



