ELLIPTIC POLARIZATION OF LIGHT BY REFLEXION. 
37 
The general result in all these cases, is that, from any one tint to another, through 
each entire order of tints, the form of the rings in the ref ected light undergoes certain 
regular changes, passing from a dislocation in one direction, to that in the opposite, 
through an intermediate point of no dislocation, or of plane polarization : and this, ex- 
hibiting a dark and a bright centred system alternately, as long as the orders of tints 
are preserved pure. 
Now these are precisely the changes in the form of the rings expressed by suc- 
cessive modifications of Mr. Airy’s formula, corresponding to the increments in the 
retardation which belong to the periodical colours of the films. 
In the instance of the metallic films it is a question whether in any case the exist- 
ence of elliptic polarization be due to the action of the film simply, or whether the 
subjacent metallic surface have any share in producing it, while the film acts as a 
retarding plate, which would render the conception of the mode of action more com- 
plex. 
These are points on which, perhaps, at present we cannot form a decisive opinion. 
But the fact that the ellipticity, to whatever cause it may be due, undergoes the 
changes just mentioned, affords an interesting comparison with theory, and may aid 
future advances towards a knowledge of the nature of the action which produces 
elliptic polarization in these cases. 
Apparatus and Method of experimenting. 
The arrangement of my apparatus was, essentially, as follows : the light was polar- 
ized by transmission through a Nicol-prism ; this was attached to a small graduated 
arc, so that it could be adjusted to throw the polarized ray at any required incidence 
on the surface under examination ; and could be turned about its own axis, so that 
the plane of polarization might be inclined to that of incidence. 
After reflexion, the ray was received by an analysing apparatus, containing a plate 
of calc-spar and a tourmaline, capable of a corresponding adjustment to different in- 
clinations, by which were exhibited the polarized rings in the several modifications 
they underwent in different cases. This part had also a motion about its own axis, 
measured by a graduated circle. It will hardly be necessary to state more details as 
to the construction of the apparatus, except perhaps to observe that an arrangement, 
by which the surface under examination could be slid horizontally under the polar- 
izing apparatus, was necessary when the object was to examine the changes presented 
in passing from one part of some surfaces to another. 
In many cases where the reflexion from parts of a surface of varying character was 
to be examined, I fitted to the eye-piece a lens of short focus, between the calc-spar 
and the reflecting surface, which enabled me to see with great distinctness the rings 
in light from very small portions of the surface, which could be isolated by covering 
the rest. 
In all cases the analyser is supposed in the position to give the dislocated rings with 
