Intelligence and Miscellaneous Articles. 169 
formed) give results something similar to those noticed on the egress 
ofa polarized ray of light after it has traversed at right angles a po- 
lished plate of quartz of a given thickness, and cut perpendicularly to 
the axis of double refraction. For it is well known, that when a ray 
has thus passed through, if examined by an analysing eye-piece, it 
exhibits a variable coloured central aperture with rings, the tints of 
the aperture changing according to the position of the polished cry- 
stalline plate with regard to the eye-piece. 
So far as the writer was enabled to experiment with his globe he 
found the results capricious, for he never could predict what would be 
the precise colour of the first halo, but the mean of a number of ex- 
periments gave the order of changes as above related. 
Mr. Lipkens favoured the writer in French with his ideas on the 
subject, which may be translated thus: The cause of the phenomenon 
presented by the experiment is evidently due to vapours formed in 
the glass globe by a change of temperature in the air which it in- 
closes, and which takes place, as we know, every time that there is a 
rapid change in the tension of that air. 
The variety of the colours of the haloes which are obtained by al- 
lowing the compressed air to escape successively in the way described, 
might depend on the dimensions of the vesicular molecules of the 
water, or on the thickness of the envelopes of these molecules. Mr. 
Lipkens then states, that as far as he knew, the necessary means 
required to obtain one centra! colour rather than another, are not 
yet discovered, and consequently it appears that were philosophers 
to discover and give a general explanation of the phenomena in ques- 
tion, their time would not be misemployed. 
London, Jan. 27, 1836. F. W. 
THULITE AND STROMITE. 
In a list of minerals appended some years since to a volume on 
crystallography, I gave 92° 30’ and 87° 30’ as the angles of this 
mineral; and I did so upon the authority of some fragments of a red 
mineral received by Mr. Heuland from Sweden, having ‘“ Thulite” 
written by Ekeberg on the paper containing them. But I have since 
found that the fragments I measured were bisilicate of manganese, 
or, as I have named it, Strdémite, of which, or of thulite, I had not at 
that time seen any other specimen. I shall feel obliged to the Editors 
of the Phil. Mag. if they will insert this correction. H. J. Brooke. 
ON THE MIRAGE, AS SEEN IN CORNWALL. 
To the Editors of the Phil. Mag. and Journal of Science. 
GENTLEMEN, 
The mirage is so frequently seen in this hilly county, that I did 
not think of sending you the following notice of one which I lately 
saw, until I recollected that some eminent travellers who had wit. 
nessed similar phenomena abroad have deemed them worthy of a 
full description in their narratives. 
Third Series, Vol. 8, No. 45. Feb. 1836, Tr 
