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On the Multiplication of Images, and the Colours which accompany 

 them in some Specimens of calcareous Spar. By David Brewster, 

 LL.D. F.R.S. Land, and Edin. In a Letter addressed to the Right 

 Hon. Sir Joseph Banks, Bart. G.C.B. P.R.S. Read June 8, 1815. 

 [Phil. Trans. 1815, p. 270.] 



The phenomena which the author here endeavours to explain, he 

 says, were first noticed by Prof. Robinson of Edinburgh, and pub- 

 lished by Mr. Benjamin Martin in his Treatise on Iceland Crystal. 

 It would not be easy to describe the appearances without reference 

 to the figures that accompany the paper. The peculiar specimens 

 of calcareous spar in which they occur, have been supposed to have 

 fissures, with a disposition to split in the direction of the longer dia- 

 gonal of the rhomboid. But the author considers it as an interrupting 

 stratum, since it appears to him not to bear the most remote resem- 

 blance to a fissure, but resembles a vein or film uniting the two pris- 

 matic portions of a rhomboid. Neither are the phenomena produced, 

 when two surfaces artificially polished in the direction of the sup- 

 posed plane are applied to each other ; but they are produced when 

 a thin film of sulphate of lime is interposed as a vein between two 

 such portions of Iceland spar, and the surfaces are closely united by 

 a cement of strong refractive power. 



The supposition of such a crystallized vein, serves also to explain 

 the varieties observable in the characters of the phenomena in dif- 

 ferent instances, which must vary according to the thickness of the 

 vein, and according to the position of its polarizing axis. 



The author also describes certain new instruments for exhibiting 

 complementary colours produced by polarized light ; but the descrip- 

 tions require the aid of his figures to render them intelligible. 



A Series of Observations of the Satellites of the Georgian Planet, in- 

 cluding a Passage through the Node of their Orbits ; with an intro- 

 ductory Account of the telescopic Apparatus that has been used on 

 this Occasion ; and a final Exposition of some calculated Particulars 

 deduced from the Observations. By William Herschel, LL.D. F.R.S. 

 Read June 8, 1815. [Phil. Trans. 1815,^. 293.] 



Since in the examination of objects so minute and so distant as 

 these satellites, it is necessary not only to magnify for the purpose of 

 removing them from the body of the planet itself, but also to collect 

 as much of their light as may be, in order to render the impression 

 they make on the eye sensible, no instrument less than a 20-feet te- 

 lescope is sufficient for discerning them ; and even with an instrument 

 of this description it is but with a favourable atmosphere, and through 

 a small part of their orbit round the primary, that Dr. Herschel has 

 been able to make his observations ; and he has also occasionally 

 availed himself of the greater light that is to be obtained by concave- 

 eye-glasses, notwithstanding the smallness of the field of view, and 

 other objections to their use. The magnifying powers employed on 



