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XLII. On the Jffecthns of Light transmitted through cry- 

 stallized Bodies. By David Brewster, LL.D. F.R.S. 

 Edin. and F.S.J. Edin. In a Letter to Sir Humphrt 

 Davy, LL.D. F.R.S.* 



Dear Sir, — In a former paper f on '^^Some Properties of 

 Light," which I took the liherty of addressing to you, and which 

 the Roval Society hoiioured with a place in their Transactions, 

 I attempted to ^ive a brief abstract of a set of experiments on 

 the Properties ot transparent Bodies in refracting, dispersing, 

 and polarising the Rays of Light. An account of the instru- 

 ments and n.tthods employed in these experiments has since 

 that time been published in my " Treatise on new philosophical 

 Instruments." 



From the general object of these researches, however, I have 

 been allured into a new field of inquiry, by the discovery of a 

 singular property of light transmitted through the agate, and 

 the prosecution of the views which it suggested has led to some 

 very extraordinary results, which, while they seem to conduct 

 us into the very mysteries of physical optics, exhibit at the same 

 time a series of appearances which far surpass, both in splendour 

 and variety, all the phaenomena of light under its usual trans- 

 formations. In again soliciting you to communicate these ob- 

 servations to the Royal Society, I tVust I need offer no apology. 

 They are closely allied with that science which you have so widely 

 extended by the most profound and brilliant discoveries ; and 

 it is probajjly from the cultivation of this department of physics, 

 that philosophy will be enabled to unfold the secrets of double 

 refraction, to explain the forms and structure of crystallized 

 bodies, and to develop the nature and properties of that ethereal 

 matter, which, while it enlivens all nature by its presence, per* 

 forms also a capital part in the operations of the material world. 



The different sul>jects of which 1 mean to treat in the follow- 

 ing letter may be included under five heads. 



L Oi! the polarizing power of the agate. 



II. On the structure of the agate as connected with its op- 

 tical properties. 



III. On the peculiar colours exhibited by the agate. 



IV. On the depolarization of light. 



V. On the elliptical coloured rings produced by obliquely de- 

 polarizing crystals. 



I. On the polarising Power of the ^gate. 

 I have already shown, in a former paper, that a ray of light; 



* From the Pliilosophicnl Tr.insactions for 1814, part i. 



t Phil. Trans, lor tlie year 1813, n. 101.— Phil. Mag. vol. xlii. p. 286. 



R 3 transmitted 



