393 
Natural Philosophy. — Optics . 
but particularly 1818, p. 259,-267 ; — Edinburgh Transactions 
vol. viii. p. 353, and this Journal, vbl.vii. p. 178), from the perfect 
similarity between the phenomena of heated and of compressed 
glass, and those of natural crystals, not only in producing the same 
polarised tints, but in exhibiting the phenomena of moveable 
polarisation^ that the glass possessed double refraction, and 
that the cause of its not being seen was the small angular se- 
paration of the images. This conclusion he considered as 
demonstrated, when he found not only that the doubly refracting 
force in regular crystals varied in intensity, and in character, pre- 
cisely as the polarising force varied ;l)ut that he could communicate 
to glass the form and structure which produced single positive 
axes, or single negative axes of polarisation, or double rectangular 
axes of polarisation ; and, especially, when he had proved 
that the polarised tints in glass could be actually computed 
a priori , by the very same mathematical formulas, mutatis 
mutandis , by which all the phenomena of polarisation in re- 
gular crystals, with one and two axes, had been brought under 
the dominion of calculation. 
This opinion, though supported by such invincible proofs* 
and even rendered necessary by Dr Young’s beautiful law of 
interference, was not only opposed, but ridiculed by some French 
philosophers. It has now, however, been established by direct 
observation. M. Fresnel, of the Academy of Sciences, whose 
labours we have so often spoken of with the highest praise, has 
distinctly rendered visible the two doubly refracted and opposite- 
ly polarised rays in glass when compressed, and Dr Brewster 
has done the same in glass that has received the doubly refract- 
ing structure from rapid cooling. M. Fresnel’s experiments are 
described in the Bulletin de la Societe Mathematique for Sep- 
tember 1822, p. 139. 
4. Mr Ramage's new Reflecting Telescope of a large Size. 
Mr Ramage of Aberdeen has finished the speculum of a new re- 
fleeting telescope, fifty-three feet in focal lengths The 
diameter of the large speculum is twenty inches. 
5. Prize offered for the best Theory of Haloes, ' The 
Royal Academy of Sciences of Prussia have offered their prize of 
fifty ducats, for the memoir which gives 66 A complete mathe- 
