38 
PROFESSOR POWELL’S OBSERVATIONS ON CERTAIN CASES OF 
the nearest approach to the dark cross with the plain metal ; the circular systems 
alone being precisely described as dark or bright centred. 
The direction of dislocation is distinguished by the quadrants in which the dark 
patches near the centre occur. The position of the line joining them, upon plain 
metal, is taken as the zero for comparison in other cases. 
The observations were repeated at several different incidences, but for the purpose 
of the comparisons here in view, it suffices to give the results at one incidence, the 
relative appearances at others being similar. They are arranged in a tabular form in 
each case. 
Observations. — Professor Forbes’s Mica. 
The original observations of Sir D. Brewster were confined to pure metals and a 
few metallic ores ; in all which the ellipticity is insensible at incidences less than 
about 30°, and comes to a maximum at between 70° and 80°. 
Besides these, as far as I am aware, the only instance is that announced by Pro- 
fessor Forbes* of elliptic polarization in the reflexion from mica when reduced to the 
particular state in which he used it for his experiments on heat. 
On repeating the experiment I observed that it has its maximum at an incidence 
between 20° and 30°, and the direction of dislocation, 90°. But the films thus formed 
do not lose their crystalline structure or retarding property: it may therefore be 
doubtful how far the effect may be explicable in the ordinary way. 
Decomposed Glass. 
In some specimens of glass, whose surface is in a well-known peculiar state of decom- 
position, not only iridescent, but having a singularly metallic lustre, I found elliptic 
polarization, though none was perceptible in other specimens, however highly irides- 
cent, which had not the metallic appearance. There are, however, anomalies with 
other metallic reflexion, for the maximum effect appears at small incidences (about 
30° or 40°), and the direction of dislocation is 90°. 
Minerals, 8$c. 
Among a variety of metallic ores, I have found elliptic polarization produced only 
by a few having a decided metallic lustre. What proportion of metal may be ne- 
cessary to give ellipticity, is an interesting question for future research. But one 
remarkable instance is that of plumbago, which gives a small though distinct ellipti- 
city ; while its composition is well known to be doubtful : but on the highest estimate 
it contains 95 of carbon to 5 of iron-j'. 
In some ores exhibiting a natural iridescence, the results seem analogous to those 
about to be described in artificial surfaces of this kind. 
* British Association, 1839, Sectional Proceedings, p. 6. 
f Thomson’s Chemistry, i. 396. 6th Ed. 
