26 DIFFICULTY OF OBTAINING GLASS 



of the arts of life ; for every improvement in navigation, espe- 

 cially to an insular people, 



" Whose march is o'er the mountain wave, 

 Whose home is on the deep," 



tends in a direct manner to increase the value of every spe- 

 cies of manufacture, by facilitating its diffusion over the globe; 

 and indirectly, in ways which are too many and too obvious to 

 admit of or require our particularizing them. 



One other example, still more recent in its history, must 

 conclude my present illustrations of the importance of scientific 

 knowledge in the arts. This relates to the improvements which, 

 called for as well as accomplished by Science, have been ef- 

 fected in the manufacture of glass for optical purposes. 



Perfect as is the manufacture of glass for all ordinary pur- 

 poses, and extensive as is the scale upon which its production 

 is carried on, there is yet scarcely any substance of artificial 

 production, in which it is so difficult to unite all the qualities 

 which are required, to satisfy the wants of science, and of 

 those arts which are directly connected with it. The general 

 transparency, hardness, sensibly-unchangeable nature, and 

 varied powers of refracting and dispersing the rays of light, 

 possessed by this substance, render it a most important agent 

 in the hands of the philosopher who is engaged in investiga- 

 ting the nature and properties of light. When, however, he 

 desires to apply it, according to the laws he has discovered, 

 in the construction of perfect instruments, and especially in 

 that of the achromatic telescope, it is found liable to certain 

 imperfections, not essential indeed to its constitution, but 

 almost always arising in its manufacture, which frequently 

 become fatal to its use in such instruments. These imperfec- 

 tions interfere so greatly with the application of the glass, and 

 are so difficult to avoid, that the progress of science is often 

 impeded by them, by the difficulty which arises of construct- 

 ing perfect instruments of great power. For example : Mr. 

 Dollond, one of our first opticians, has not been able to ob- 

 tain a disc of flint-glass four inches and a half in diameter, fit 

 for the object-glass of a telescope, within the last five years, or a 

 similar disc five inches in diameter, within the last ten years. 



On this account the object-glasses of powerful telescopes 

 not only constitute by far the most expensive part of them, 

 (several hundred pounds and even a greater sum having been 

 paid for a single lens), but render the entire cost of such in- 

 struments so nigh, as to remove them beyond the means of 

 many persons, who are equally desirous, and well qualified, to 

 use them, with success and advantage, in the promotion of sci- 

 ence. From the difficulty in the cultivation of astronomy thus 



