326 Mr Haidiiiger on Davyne^ 



that period of life when the eye begins to feel the approach of 



age. 



Mr Airy has misconceived the information which he has re- 

 ceived, " that a foreign artist has made spectacle-glasses with 

 cylindrical surfaces of different radii for general use." This 

 information is quite correct, and we had one of these lenses 

 executed for us many years ago by a Scotch artist, which is 

 figured and described in the article Optics in the Edinburgh 

 Enci/clopcedia, vol. xv. p. 509- The cylindrical surfaces, 

 however, are placed with their axes, or homologous lines, trans- 

 verse to each other, so that the whole lens acts like a spherical 

 lens. Mr Airy, consequently, is quite correct in his conclu- 

 sion, that these lenses were not intended for correcting any 

 such defect as that which he has observed, and so well de- 

 scribed. 



Aet. XXIII. — On Davi/ne, a New Mineral Species. By Wil- 

 liam Haidingee, Esq. F. R. S. E. &c. Communicated by 

 the Author. 



The Prodromo della Mineralogia Vesxiviana^ by Messrs Mon- 

 ticelli and Covelli, has been so frequently quoted of late, and 

 accounts and abstracts of it given in systematic works and in 

 journals, that a notice of it might have been ere now expected 

 also in the present ^Journal of Science. A great many species 

 were described in that work, some of them probably new, but 

 with so little regard to the present state of mineralogical infor- 

 mation in the rest of the world, have they been brought for- 

 ward, that mineralogists will have to adopt them only with the 

 utmost caution, if they wish to avoid giving double descrip- 

 tions of the same substances. 



Christianite, for instance, one of the species proposed as new, 

 was described under the name of Albile long previous to the 

 publication of the Prodromo, and much better determined by 

 Professor GustavusRose, who published his masterly account 

 of it in Gilbert's Annals for February 1823. It is much to 

 be regretted that names so honourable and respected as those 

 of his Royal Highness Prince Christian of Denmark, and Pro- 



