Permanent Colours of Opake Bodies, 149 



Subflances, are very numerous, and the aclion 

 and powers of fuch bodies, in refleding and re- 

 frafling light, have been, in a great meafure, 

 difcovered and explained. 



In confequence of thofe difcoveries, optical 

 inftruments have been brought to a confiderable 

 degree of perfedion. 



It is further obfervable, that from the mifcon- 

 ception of a fingle ciicumftance, in this depart- 

 ment of the fcience, there arofe an obftacle to 

 the conftrudion of telefcopes, which the united 

 efforts of artificers were unable to furmount, till 

 the error was rectified. 



This error, which had efcaped even the pene- 

 tration of Sir Ifaac Newton, originated from a 

 fuppofition, that the divergency of all the rays, 

 which pafs through any media, is conftantly 

 equal to their refraflion. 



The inaccuracy of this opinion was firft ob- 

 ferved by M. Euler, and the fubjed was after- 

 wards attentively purfued by the Academicians 

 Clairaut, Klingenftierna, Beguelin, Zeiher, and 

 D'Alembert : but, we are indebted to M. Dol- 

 land, for the final difcovery of the Principles, 

 and their application to the conftrudion of tele- 

 fcopes, fuperior to any which had ever been 

 before produced. 



In this happy inftance, the genius of the Phi- 

 lofopher, and the hand of the i\rtift, were united 

 in the fame perfon. 



L 3, I have 



