ADDRESS. lvii 
_ The vibrations of a polarized ray are all parallel to a fixed direction in the 
plane of the wave; but that direction may be either parallel, or perpendt- 
cular to the plane of polarization. In the original theory of Fresnel the 
latter was assumed to be the fact; and in this assumption Fresnel has been 
followed by Cauchy. In the modified theories of MacCullagh and Neumann, 
on the other hand, the vibrations are supposed to be parallel to the plane of 
polarization. This opposition of the two theories was compensated, as re- 
" spects the results, by other differences in their hypothetical principles ; and 
both of them have led to conclusions which observation has verified. There 
seemed, therefore, to be no means left to the theorist to decide between these 
conflicting hypotheses, until Professor 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 light is transmitted 
through a fine grating, it is turned aside, or diffracted, according to laws 
_ which the wave-theory has explained. Now Professor 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 relation. It only remained, therefore, for ob- 
servation to determine whether the planes of polarization of the incident 
and refracted rays were similarly related, or not. The experiment was 
undertaken by Professor 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 Professor Stokes to the difference in the nature of the 
3 gratings employed, the substance of the diffracting body being supposed to 
exert an effect upon the polarization of the light, which is diffracted by it 
_ under a great obliquity. I learn from Professor Stokes that he proposes 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, unfortunately, greatly complicate the dynamical theory of 
light. In the meantime the hypothesis is one of importance in itself, and 
_ deserves to be verified or disproved by independent means. I would venture 
suggest that it may be effectively tested by means of the beautiful Inter- 
oS =. 
study the effects upon light produced by grazing a plate of any soluble sub- 
stance enclosed 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 overthrown 
the decisive at soar sancore is erucis of MM. Fizeau and Foucault. It was 
th eory—would be found wanting. Its truth has recently been put to an ex- 
perimental test by M. Jamin. Water, it is known, has its maximum of 
aa density at about 40° of Fahrenheit ; so that, if Newton’s law were true, its 
