192 Proceedings of the Royal Society of Edinburgh. 



Phosphate of lime with a little oxide of iron. 85.08 83.31 



Carbonate of lime, 10.78 15.11 



Silica, .... . . 0.34 0.2!) 



Bituminous matter, . . . 3.96 1.47 



100.2 100,00 



Theproportion of phosphate of lime, appears thus to be pretty uni- 

 formly 5 6ths of the whole. The variation in the proportion of car- 

 Ijonate of lime may probably be influenced by tlie matrix, from which 

 also the bituminous matter' is derived. The limestone, when dissolv- 

 ed in muriatic acid, leaves a dark bituminous matter, in the propor- 

 tion of 2.5 per cent. 



2. Notice relative to the Polyzonal Lenses belonging to 

 the Commissioners of the Northern Light-houses. By- 

 Alan Stevenson, Esq. 



These lenses, which were exhibited to the Society by Mr Steven- 

 son, are three in number. One, a plano-convex lens, two feet six 

 inches square, was made by M. Soleil at Paris, under the superinten- 

 dence of the late M. Fresnel. Another is a double convex circular 

 lens of flint-glass, three feet in diameter, which was constructed by the 

 Messrs Gilbert of London, at the suggestion of Sir David Brewster. 

 The third is a circular plano-convex lens, two feet six inches in dia- 

 meter, cast in one piece as originally proposed by Buffon. This lens 

 has been executed for the first time by the Messrs Cookson, plate- 

 glass-makers, Newcastle. 



The author, with the assistance of Mr John Adie jun., made a 

 numerous set of observations, to determine the relative value of these 

 lenses, and for this purpose ascertained, ^>,5f, the mean focal distance 

 of the central lens, and the several concentric rings of each, conipa- 

 rino- the results with the focal distance of its aggregate surface ; and 

 secondly, the mean diameter of the spectra produced in the focus, by 

 the central lens and several zones separately, comparing this result 

 also with the spectrum foumed by the whole compound lens. The 

 general results are as follows : — 



FRENCH. Focal Dist. Diam. of Spec. 



Feet. In. In. _ 



Mean for central lens and 5 rings severally, . 2 11.C5 O.OC 

 Aggregate surface of whole lens, . . .3 0.25 O.7O 



Diflference, O.CO 0.04 



NEWCASTLE. 

 Mean for centraMens and 5 rings severally, . 2 11.97 0.5? 

 Aggregate surface of whole lens, . . .3 0.75 0.75 



Difference, O.78 0.18 



LONDON. 

 Mean for central lens and 4 rings severally, . 3 1.90 0.84 

 Aggregate surface of whole lens, • • 3 3.00 1.25 



DifTerence, l.IO 0.41 



