Light from Transparent Matter. 83 



must be known before further progress (other than tentative) 

 can be made. From the fact that in all the known gases A is 

 independent of the nature of the gas, Green argues that we 

 may assume the same for B, " at least when we consider those 

 phenomena only which depend merely on different states of the 

 same medium, as is the case with light w — an inference which 

 certainly appears very precarious. In a note he says, " Though 

 for all known gases A is independent of the nature of the gas, 

 perhaps it is extending the analogy rather too far to assume 

 that in the luminiferous sether the constants A and B must 

 always be independent of the state of the sether as found in 

 different refracting substances. However, since the hypothesis 

 greatly simplifies the equations due to the surface of junction 

 of the two media, and is itself the most simple that could be 

 selected, it seemed natural first to deduce the consequences 

 which follow from it before trying a more complicated one, and^ 

 as far as I have yet found, these consequences are in accordance 

 with observed facts." 



In a very wild criticism of this theory, at the end of an other- 

 wise sound paper *, Kurz, having mistaken the meaning of A, 

 B, attributes to Green the absurd assumption that the wave- 

 velocities are the same in the two media, and metaphorically 

 holds up his hands in amazement. I need hardly point out 

 that Green's conditions A=A,, B = B ; are something quite 

 different, and imply simply an identity of statical properties in 

 the case of the two media. It may be shown, however, that the 

 first (A=A,) is unnecessary, a fact which Green does not seem 

 to have perceived. The cause of the refraction is a variation of 

 the dynamical property (density) . The rest of Green's reason- 

 ing is rigorous, admitting of no cavil. When the vibrations 

 are normal to the plane of incidence, the amplitude of the 

 reflected vibration is expressed accurately by FresnePs sine-for- 

 mula ; but the tangent-formula is only applicable to vibrations 

 in the plane of incidence as a first approximation. It is evident 

 that, in order that theory may at all agree with observation, the 

 vibrations of light must be supposed to be performed nor- 

 mally to the plane of polarization ; indeed the two assumptions 

 of constant rigidity and normal vibrations are closely bound 

 up together in all parts of optics. The effect of the hypotheti- 

 cal relations A = A ; , B=B y is greatly to simplify the bounding 

 conditions which then express the equality of the component 

 displacements and their derivatives on the two sides of the sepa- 

 rating surface. In this form they become identical with the 

 so-called Principle of Continuity of Movement stated by Cauchy, 



* Pogg. Ann. vol. cviii. p. 396. 

 G2 



