40 
PROFESSOR POWELL’S OBSERVATIONS ON CERTAIN CASES OF 
gan to appear on the upper surface, and assumed the form of rings round the point 
of application of the heat. 
Each tint appeared in succession, first at the centre, and thence extended itself. 
The first tints succeeded each other in the course of a few seconds ; but the latter 
more slowly ; until at length, after the heat had been continued for a considerable 
time, no further change took place. 
The tints, proceeding in order from the outermost to the centre, may be described 
and grouped as follows : — 
1. Reddish brown, crimson, deep purple, dark blue, light blue, faint yellow. 
2. Faint red, light blue, pale reddish brown (at the centre). 
The last two colours were very faint, and seemed covered with a sort of cloudy 
whiteness. 
It is difficult to compare these tints with any part of Newton’s scale: but I have 
here grouped them in two divisions which seem clearly marked by a periodicity, 
while in the last two tints a peculiar character appears to prevail. 
The supposition of a change in the degree of oxidation before referred to, seems to 
accord with the peculiar appearance of the later tints, while the periodicity of the 
former agrees with the supposition of thin plates up to that point. 
In the observed phenomena of ellipticity there appear distinctions corresponding 
to the regular orders of tints ; for in passing them in succession under the apparatus, 
corresponding changes in the amount and direction of dislocation occur, as far as the 
red of the 2nd set ; while in the remaining part, the ellipticity is greatly diminished, 
and no change in direction takes place. 
The figures 1, 2, &c. distinguish successive parts of each coloured space at which 
the changes occur. 
Incidence. 
Surface. 
Polarization. 
Centre. 
Direction of 
dislocation. 
•<r 
o 
o 
Steel, plain .... 
Elliptic, large 
0 ° 
Films by heat. 
/ ■) 
1st Red 
Blue 
Yellow 
Elliptic, large .... 
Plane 
Elliptic, large .... 
Plane 
Bright. 
Dark. 
0 0 
0 0 
2nd Red 
Blue, &c 
Elliptic, large .... 
Small 
Dark . . 
0 0 
0 0 
Coloured Films on Copper by Heat. 
I formed colours in exactly the same way on plates of copper of the same size. 
The tints here do not appear exactly the same as those on steel, though there is some 
general correspondence : they follow this order, commencing from the outermost. 
1 st. Red, purple, pale yellow, yellow. 
2nd. Red, green, dull brown. 
