ELLIPTIC POLARIZATION OF LIGHT BY REFLEXION. 
41 
The last two tints have a sombre appearance, different from the metallic lustre of 
the others : this (as in the last case) may accord with the supposition of a different 
degree of oxidation. 
Here also I find similar changes in the ellipticity through the first tints, but none 
in the last two. 
Incidence. 
Surface. 
Polarization. 
Centre. 
Direction of 
dislocation. 
o 
1 o 
Copper, plain . . 
Elliptic, large 
0° 
Films by heat. 
yi 
( -i 
1st Red 
Elliptic, large .... 
Plane 
Elliptic, large .... 
Plane 
0° 
Purple 
Yellow. ... 
Bright. 
Dark. 
90° 
2nd Red 
Elliptic, large .... 
Small . . 
0° 
Green, &c 
Dark . . 
0° 
Nobili’s Coloured Films. 
Besides furnishing me with several specimens of these films on steel plates, in 
which very brilliant tints were developed as far as the 3rd order, Dr. Daubeny kindly 
caused other plates to be prepared in his laboratory, in order to trace the order of 
the effects produced. 
In forming one of these, the process was continued longer than before, and thus a 
4th order of very faint colours succeeded. In another instance the process was 
continued still longer, and colours were formed which went through the same series 
as before, but at the 4th order they became extremely dusky and obscure ; and the 
5th order was barely visible from increasing opacity. They followed one another in 
succession towards the circumference, where they were at length crowded together, 
forming a narrow fringe, within which the central space was of a deep brown or 
black, of a dull, opake, appearance ; and when this had occurred, the galvanic action 
being kept up for some time afterwards, no further change took place. 
It can hardly be doubted that the increasing dulness in the later orders of tints, 
and final opacity of the film, are due to some change in the nature of the deposit which 
is superinduced at this stage of the process. 
On bringing the coloured parts of the plate successively under the apparatus, the 
changes in the rings were clearly marked through the three orders of tints; though 
they followed in very close succession, commencing upon the edge of the film ; and 
in the passage through the bright-centred system the variations of colour were 
striking, and at first somewhat perplexing. 
The following general results were derived from repeated observations with dif- 
ferent plates compared together. 
MDCCCXLIII. G 
