232 Dr Pearson, On the use of Quartz or Rock-crystal [May 1, 



terrestrial objects; and I think that the only real advantage to be 

 gained by the use of this material is from its transparency. I am 

 not sure how far this advantage in the present state of glass manu- 

 facture will be actually realized. Rock-crystal is undoubtedly a 

 most beautifully transparent substance, but so is the best modern 

 glass ; and when employing alternately the old and actually green- 

 ish, though I must say well-made, object-glass, and the new one 

 compounded with rock-crystal on objects varying in distance from 

 half-a-mile to ten and fifteen miles, I was unable to say that one 

 showed more illumination than the other. It is true that in an 

 erecting eye-piece there are four lenses, in this case all of glass, so 

 that altogether the improvement is only one-sixth instead of five- 

 sixths (because the flint-glass must always remain in the object- 

 glass), of that due to the actual difference in transparency between 

 crown-glass and quartz ; but I own that I expected more. At the 

 same time I do think that a small land telescope, say of 1^ inch 

 object-glass, like my own, with all the lenses in the eye-piece of 

 quartz, would be perhaps an expensive but certainly a very perfect 

 instrument 1 . 



Since I read this paper, Prof. Stokes has kindly drawn my 

 attention to the complete table of the spectrum for quartz given by 

 Rudberg in Poggendorf, 1828, vol. xiv. (and also in Miller's Miner- 

 alogy), as well as to a paper of his own on a kindred subject in the 

 Proceedings of the R. S., 1877. If we combine Fraunhofer and 

 Rudberg's results for the three substances we are discussing, viz. 

 flint-glass, crown-glass and quartz, and compare them with those 

 given by Brewster, we find that while the first two experts make 

 the dispersive powers '068, "039, '032, the latter makes them *052 

 (about), - 033, "026, from which it is clear that the dispersive ratios 

 between the first and second and the first and third are about the 

 same, whichever authority we follow ; but that the radii of the sur- 

 faces of the lenses will be considerably affected by the use of quartz, 

 e.g. if crown-glass is used, in a compound lens of 10 inches focal 

 length, the focal length of the flint lens ought to be about 7'63 

 inches, and that of the crown-glass lens 433 inches ; while the lens 

 compounded with quartz will require a focal length of 10'8 inches 

 for the flint-lens, and 5 "2 inches for the lens actually cut from 

 quartz : the radii of the surfaces being correspondingly modified 

 according to the table. 



I have also obtained a copy of M. Cauchoix's patent, dated 

 Paris, July 7, 1828. In it he simply says that rock-crystal being 



1 Herschel (Neio Edinb. Phil. Mag. vi. p. 367), stating the dispersive ratio of 

 flint- and crown-glass to be 0-567, gives a table and rules for estimating the proper 

 focal lengths of the crown and flint lenses for an achromatic object glass of an 

 assumed focal length of 10 inches. Fraunhofer's dispersive ratio is about '577 for 

 the same substances. 



