4 REPORT—1846. 
It is certain however that in a great number of cases of thin plates examined by 
the author of this communication no ellipticity can be detected. Glass superficially 
decomposed and giving brilliant tints produces no ellipticity, except in those instances 
where it has a decided metallic lustre. Vapour condensed on soaped glass (in the 
manner described by Sir D. Brewster), oils of turpentine, cassia, &c. between glass 
plates (the upper being slightly prismatic to separate the reflexions), are equally de- 
void of any indication of ellipticity. 4 
The theory therefore clearly needs some further modification to express the condi- 
tions under which the effect may be sensible. _ 
There are doubtless many cases of thin plates in which elliptic polarization is pro- 
duced (as in the films formed by Nobili’s process and by heat, as investigated by the 
author of this communication, or again, as in mica which has become laminated, &c.), 
but in these cases the modus operandi is well understood; the former arising from the 
enormous refractive power, in the latter from the crystalline structure. 
In the instance of China ink observed by the author, the ellipticity appears equally, 
whether it be in the form of a film or in a solid mass; but it is only seen in the 
purest specimens. 
In the numerous other cases examined by Mr. Dale it does not appear that any- 
thing like films can be supposed; the only condition seems to be the high refractive 
ower. 
It may still be a question, then, whether the theory proposed independently by M. 
Cauchy and by Mr. Tovey be not more easily applicable; since it requires nothing 
but the very simple and admissible hypothesis, that the molecules of ether, for a minute 
depth within the surface, are unsymmetrically distributed*, 
In various substances containing but a very small proportion of metal, ellipticity 
has been detected, in addition to those enumerated by the author on a former occa- 
sion. Among these are prussian blue, and a specimen of the meteorite from the Cape 
of Good Hope, 1839, which contains only about 33 per cent. of protoxide of iron, very 
small portions of oxides of nickel and chrome, and a minute trace of metallic iron. 
On the Bands formed by partial Interception of the Prismatic Spectrum. 
By the Rev. Professor PowEtt, V.P.B.A, 
In the discussion} relative to these bands one or two points suggested themselves 
which appear to need further remark. 
The principal objection was, that according to the theoretical formula, a contrac- 
tion of the aperture of the eye or telescope should produce an enlargement of the in- 
tervals between the bands, which is not confirmed by experiment. 
The author finds that with a contraction down to the twentieth of an inch, though 
there is no sensible enlargement of the intervals, yet the bands become greatly more 
vivid and distinct, while they extend only over a smaller portion of the spectrum at a 
time §. 
With the same plate, the enlargement of the intervals appears to depend solely on 
the increase in the angular extent of the spectrum subtended at the eye, whether pro- 
duced by a greater distance from the origin, a greater prismatic angle, higher di- 
spersion, or greater power in the telescope. 
The formula involves the ratio of the semieaperture to the distance of an assumed 
point on the retina from the geometrical image of the point of light; and this “ dif- 
fusion” being no arbitrary supposition, but a direct portion of the theory, it seems un- 
reasonable to pronounce it “ untenable” and “ quite inadmissible,’ when the question 
at issue is, whether the theory as a whole will apply to the phenomena. 
Apart from all theory, when under certain conditions bands are formed equally 
whether the plates be applied at one end of the spectrum or at the other, “ polarity” 
seems an improper term by which to describe the effect, 
* See the author’s Treatise on the Undulatory Theory, &c., p. 33. 
+ See Phil. Trans. 1839, i. 86. 
t See Brit. Assoc. 1845, Sectional Proceedings, p. 7. 
§ Both these results have since been shown to be in perfect accordance with theory by 
Mr. Airy.—Phil. Mag. Nov. 1846. 
