1882.] Dr Pearson, On the use of Quartz or Rock-crystal, dr. 229 



The following communications were made to the Society. 



(1) On the use of Quartz or Rock-crystal in the object-glasses 

 of telescopes. By Dr J. B. Pearson. 



I cannot ascertain exactly how soon the rock-crystal came into 

 use for optical purposes as distinguished from the knowledge of its 

 optical properties. In the excellent Encyclopcedia Britannica of 

 1810, I find no mention of its use for spectacles, though it is said, 

 describing it under the head of Mineralogy, that "on account of its 

 lustre and transparency it is employed in jewellery, and particularly 

 when it is coloured, as those from Cairn-Gorm in the North of Scot- 

 land, many of which are held in high estimation," and its specific 

 gravity is given, 2*650 to 2'8S8. I am inclined however to think it 

 must have been so employed at that date, as Poggendorf, in a note 

 to the paper I am about to refer to, in 1829, speaks of spectacles of 

 rock-crystal as "known for a considerable time." 



In the Philosophical Transactions for 1821 there is a short paper 

 by Mr George Dollond on its use in Micrometers. 



Dr Kitchener, writing in 1824 in his two small volumes on the 

 Eye, where much information is given as to telescopes and their 

 construction, simply says, with reference to spectacles, that the 

 "notion some have as to Pebbles, that they are much cooler to the 

 eyes than glasses," is a prejudice ; and that "it is quite impossible 

 to distinguish between good Pebbles and good Glasses." 



But in Poggendorf's Annalen, 1829, vol. 15, pp. 244-57, there is 

 a translation of a paper in a French serial by M. Cauchoix, the 

 maker of the object-glass of the Northumberland Equatoreal be- 

 longing to this University, setting forth at length the advantage of 

 substituting rock-crystal for crown-glass in the composition of 

 achromatic object-glasses. He says that he had taken out a patent 

 for his invention; and then goes on to describe in detail the ad- 

 vantages he conceives may be thus obtained. After speaking of 

 the endeavours which had been made, especially by M. Guinand, of 

 Neuchatel, to increase the specific gravity and so the refractive in- 

 dex of flint-glass, a point on which there seems to have been a 

 good deal of controversy (see the long note at p. 246 in Poggendorf; 

 and the paper in the Philosophical Transactions, by Faraday, 1830), 

 he says he is surprised that no one had till then thought of substi- 

 tuting for crown-glass a substance, i.e. rock-crystal, with a higher 

 refractive index, but a lower dispersive power; especially as in 

 physical experiments and for micrometers (I suspect he refers to 

 Mr Dollond's paper) he says it had already been often so employed. 

 He does not attempt any theoretical investigations, or employ any 

 algebraical formula;, simply asserting that, taking as a standard, 

 an excellent telescope by John Dollond made in 1758 of Si inches 



