Description of Fraunhofer s Telescopes. S05 



subjects, have both abandoned the opinions they formerly 

 maintained, and that the conclusions at which they have event- 

 ually arrived are strikingly discrepant. * 



The Euchroite not only differs in mineralogical characters 

 from the other native arseniates, but is also distinct in chemi- 

 cal composition. The proportion of oxide to acid is very si- 

 milar to that of Count Bournon's third species, as analysed 

 by Mr Chenevix : * only this mineral appears to contain no 

 water of crystallization. A new analysis of the Cornwall arseni- 

 ates is at present a desideratum ; for, notwithstanding the known 

 accuracy of Mr Chenevix, that chemist seems to have disre- 

 garded the probable existence of phosphoric acid in some of 

 his arseniates. As I expect soon to possess, through the kind- 

 ness of Mr Allan and Mr Haidinger, a whole series of the 

 Cornwall arseniates, I hope in no long time to enter on the in- 

 vestigation of them. . 



Art. XXIII. — Description of Fraunhofer' 's Large Achro- 

 matic Telescopes. With a Plate. 



The great discovery of a method of making flint glass in large 

 pieces, and perfectly pure and free from stria?, which was made 

 by the late M. Guinand, and of which we have given a full 

 account in this number, (see p. 348,) may be considered as 

 forming an era in the history of the achromatic telescope. 



By means of this glass, M. Fraunhofer, the director of the 

 Optical Institute or Manufactory at Benedictbauern, near 

 Munich, has constructed achromatic telescopes far superior to 

 any that have hitherto been made ; and we can assure our read- 

 ers, of what many of them will deem incredible, that this emi- 

 nent artist can now make achromatic object glasses with an 

 aperture of eighteen inches. But it is not merely in the opti- 

 cal part of the instrument that M. Fraunhofer has been suc- 

 cessful. His various improvements on the apparatus which 

 accompanies the telescope, and his ingenious micrometers for 

 measuring angles of all 1<inds in the heavens, have received the 



• Phil. Trans. 1801, p. 19<>. 

 vol.. II. NO. IT. APRIL 1825. X 



