of the Plane of Polurization of Light. 101 
from the pole of a magnet. From the way in which he writes 
of my theory being refuted by his experiments, it is plain that 
he is unacquainted with my paper “ On the Hlectromagnetic 
Theory of the Reflection and Refraction of Light” (R. 8. 
Trans. 1880, part ii. p. 691), in which the following sentences 
occur (p. 709) :-— 
“In comparing these expressions with the results of Mr. 
Kerr’s admirable experiments, it is necessary to observe, as I 
mentioned before, that the introduction of a difference of 
phase between the reflected components is a question of a 
different order from that here discussed, and probably to 
some extent at least depends on the want of abruptness in the 
change from one medium to the other. For instance, my 
expressions give no change of plane of polarization when 
light is reflected normally from the end of a magnet, but they 
would lead one to expect that the only effect was a slight 
elliptic polarization, the major axis of the ellipse being in the 
same plane as the original plane of polarization. Now 
Mr. Kerr’s experiments show that there is some rotation 
of this plane by reflection, and a supposition similar to one 
long ago proposed to explain the known elliptic polarization 
of metallic reflection—namely, that the efficient reflecting 
surface has some depth—may easily be shown to lead to 
Mr. Kerr’s result. On this hypothesis the reflected ray is 
the resultant of the rays reflected from a small thickness at 
the surface of separation of the media; and in the case of 
normal reflection from the end of the pole of a magnet, each 
of these components would be slightly turned from its original 
plane of polarization owing to having passed through a very 
small thickness of a very powerful rotatory polarizing sub- 
stance—namely, this superficial layer of the magnet ; hence 
it is evident that their resultant would no longer be polarized 
in the same plane as the incident ray. I only give this as an 
instance of how this question of a difference of phase affects 
the results, and how the hypotheses that have been framed 
to explain it might be used to bring my results into complete 
accord with Mr. Kerr’s experiments.” | 
Prof. Kundt will see from this that I have anticipated the 
objection he raises, and have also anticipated him in pointing 
out in what direction to look for the explanation of the out- 
standing phenomena, to one of which he has called attention. 
Even in my first paper, the one with which Prof. Kundt is 
acquainted, I pointed out that it would be necessary to include 
the complications of metallic reflection in order to arrive at a 
complete theory. J cannot agree with Prof. Kundt in 
thinking that the incompleteness he has now pointed out and 
