50 



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

 the central lens and several zones separately, comparing this result 

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

 general results are as follows : — 



FRENCH. Pocal jjjst. Diam, of Spec. 



Feet. In. In. 



Mean for central lens and 5 rings severally, . 2 11.65 0.66 

 Aggregate surface of whole lens, . . .3 0.25 0.70 



Difference, 0.60 0.04 



NEWCASTLE. 



Mean for central lens and 5 rings severally, . 2 11.97 0^7 



Aggregate surface of whole lens, . . .3 0.76 0.75 



Difference, 0.78 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 



Difference, 1.10 0.41 



The conclusion at which he arrives is, that the Newcastle lens is 

 scarcely inferior to the lens made in Paris ; and that the Messrs Cooksoii 

 are undoubtedly entitled to the merit of having successfully combat- 

 ed the difficulties which attend the making of polyzonal lenses in 

 one piece, — difficulties previously considered insurmountable, but 

 which they have overcome at the first attempt. The author expects 

 that some of the particulars of the method by which they effected 

 their purpose will be communicated to the British Association at their 

 meeting at Edinburgh in September next. 



3. Additional notice relative to the Freshwater Limestone in 

 the vicinity of Edinburgh, belonging to the Carboniferous 

 Group of Rocks. By Dr Hibbert. 



In this paper, Dr Hibbert explained the progress which had been 

 made, since he first pointed out the existence of plants as well as of 

 fish and saurian animals, in the fresh-water limestone of Burdie- 

 house, towards a farther investigation of these animal remains. 



The following is a representation of the first saurian relic discover- 

 ed in the limestone of Burdiehouse by the author : — 



