2 NINTH REPORT — 1839. 



schriften der Academie der Wissenschaften zu Miinchen fur die 

 Jahre 1814, 1815, band v.). This M^as translated into French in 

 Schumacher's Asti^onomischen Ahhandlungen, Zweites Heft, 

 Altona 1823 ; and from this last an English translation was 

 made in the Edinburgh Philosophical Journal, Nos. 18 and 

 19, Oct. 1823. 



M. Rudberg (in Poggendorff's Annalen, band xiv. § 45, 

 and band xvii.) gave similar series of results for the ordinary 

 and extraordinary refractions, and those along the axes of elas- 

 ticity of several crystals. 



The importance of extending such observations vi^as urged by 

 Sir J. F. W. Herschel, in his Treatise on Light (§ llf/. and 

 1121.), in 1827. The same recommendation was repeated by Sir 

 D. Brewster, in his Report on Physical Optics to the British As- 

 sociation in 1833 (Second Report, p. 319); and with peculiar 

 force, as coming from a philosopher who had done so much for 

 the determination of refractive powers, and almost everything 

 for the dispersive powers, of a vast range of substances. 



This led to the formal recommendation of such researches 

 by the Association (Third Report, p. 473) ; and a grant was 

 placed at my disposal, for the prosecution of this object, at the 

 Dublin Meeting, 1835. I used every endeavour to carry on the 

 observations ; and though I found the difficulties in practice 

 greater than I had anticipated, I was able to present at the 

 Bristol Meeting in 1836, and circulated in print, a series of first 

 approximate determinations of indices for various media, which 

 was afterwards embodied in the memoirs of the Oxford 

 Ashmolean Society. Through the same body I published 

 "Additional Observations," &c. in 1838, containing repetitions 

 of some important measurements, with a view, not only to in- 

 creased accuracy, but to the settlement of some points which 

 seemed doubtful. Some discussion which took place on these 

 points at the Newcastle Meeting, 1838, led to a further exami- 

 nation ; and during the present year I repeated the most im- 

 portant determinations, together with some additional investi- 

 gations, which were printed by the Ashmolean Society under 

 title of " A Second Supplement to Observations," &c., 1839. 

 These are referred to in the following report under the designa- 

 tion of First, Second, and Third Series, respectively. 



The object of this Report is to bring together all the data we 

 at present possess of this kind, in a uniform tabular arrange- 

 ment, which will comprise, — 



1st. Fraunhofer's Indices. 



2nd. Those of M. Rudberg. 



3rd. Those obtained by myself, collected from my several 



