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



aperture, and 42 inches focal length, this focal length may be re- 

 duced to 36 inches by the use of Guinand's flint-glass instead of 

 that of English manufacture : while he proceeds to say that the 

 focal length, if English flint-glass be retained and combined with 

 quartz, will be reduced to 28^ inches and to 25 inches if Guinand's 

 glass be employed. As an example he specifies an ordinary tele- 

 scope of If inches aperture and 18 inches draw, which he had fur- 

 nished with an object-glass half again as broad, the length of the 

 telescope remaining the same. 



In the Astronomische Nachrichten for 1836, vol. xiii., p. 273, 

 there is a letter from M. Cauchoix, written at the request of the 

 editor, giving some of the details I have above cited from the article 

 in Poggendorf, and adding that he had supplied telescopes with 

 object-glasses constructed with quartz (he does not specify the kind 

 of flint-glass employed), which had severally (1) an aperture of six 

 inches and a focal length of 48 inches, or in the proportion of 1 to 

 8 ; (2) 4|- inches aperture, and focal length, 30 inches ; proportion 

 1 to 6'6, &c. ; (3) 3 inches, and 18 ; proportion 1 to 6 ; (4) 2.\ inches, 

 and 18 ; proportion 1 to 7 ; but he adds that the general proportion 

 he would recommend is 1 to 8, or perhaps, if special excellence 

 were desired, 1 to 10. 



M. d'Abbadie, when he went on his long geodesic expedition to 

 Abyssinia, from 1839 to 1849, during which time he returned 

 once to visit Paris, took with him two telescopes of this construc- 

 tion by Cauchoix, one of 75 mm - aperture and 0™- 90 focal length, 

 proportion 1 to 12; the other of 72 mm - aperture and m - 84 focal 

 length, proportion 1 to 11*66 ; one of these had a small finder, 

 2o mm - aperture and m ' 15 focal length, proportion 1 to 6, or much 

 higher. It was the mention of these instruments by M. d'Abbadie 

 which first drew my attention to the subject. 



In 1862, M. d'Abbadie, or his editor M. Radau, seems to endorse 

 M. Cauchoix's view as to the power of quartz to shorten a tele- 

 scope, as he says, "l'avantage du crista! de roche est de diminuer 

 la longeur de la lunette, longeur fort incommode en voyage." It 

 may be that he relied on his maker, and had not examined the 

 question for himself. 



I have never seen rock-crystal mentioned anywhere else as used 

 in the construction of object-glasses; but feeling curious on the 

 subject, I employed Mr Hilger to make me one for an old telescope, 

 intending to compare the two object-glasses together. He has done 

 so, but though he assures me that the radii of the lenses were cut 

 according to the rules supplied him by a well-known gentleman, 

 whose name I do not give, but which I am sure would be perfectly 

 satisfactory to all here present, I was much surprised to find that 

 the crystal lens has a focal length only one half of an inch in a 

 foot, or one twenty-fourth shorter than my old object-glass, which. 



