ELLIPTIC POLARIZATION OF LIGHT BY REFLEXION. 
39 
Oxidation. 
The common effect of oxidation is to reduce the elliptic into plane polarization. 
This is the case with the oxidation produced on copper, &c. by a drop of dilute acid, 
and with the dull tarnished surface of most metals after long 1 exposure. 
A transparent film on the surface of polished metal may diminish or destroy the 
ellipticity, obviously from its refraction causing- the rays to fall on the metal beneath 
at too small an angle. 
Mercury, when pure, gives a large elliptic polarization ; and even when the surface 
is coated with the film of oxide which so readily forms upon it, the ellipticity remains 
unaltered : the surface of the oxide, however, has a sort of metallic appearance. 
Daguerreotype Plates. 
With one of these plates on which a picture had been formed, at incidences of 60^ 
and 70 °, I could perceive no difference in the degree of ellipticity, or direction of dis- 
location, between those parts of the surface which remained bright, and those which 
had been most powerfully acted upon. 
Tempered Steel. 
With reference to the same objects I tried plates of steel in its ordinary state, and 
in two stages of tempering, viz. the yellow or straw-coloured, and the blue. The 
former is well known to be that formed at the lowest heat. The film or state of sur- 
face thus produced occasions changes both in the amount and direction of the dis- 
location. 
Dr. Thomson*, in describing the process of tempering, observes that it is a question 
whether the changes in colour be due to thin plates of an oxide simply, or whether 
there may not be different oxides produced in succession. 
This last opinion agrees with the nature of my results, as well as with the absence 
of change of colour on altering the inclination. The results are as follows : — 
Incidence. 
Surface. 
Polarization. 
Centre. 
Direction of 
dislocation. 
70° 
Steel, plain .... 
Elliptic, large .... 
0° 
Steel, tempered, 
w 
< ^ 
Yellow | 
Blue 
Elliptic, very small, 
or none 
Very small 
j- Dark... 
Dark... 
CO 
O O 
o o 
Coloured Films on Steel by Heat. 
lo examine the phenomena more precisely in their order of succession, I formed 
coloured films on plates of highly polished steel, about five inches square, by applying 
to the under side, at the centre, the flame of a spirit-lamp, when colours soon be- 
* Chem. i. 386. 6th Ed. 
