REFRANGIBILlTr of LIGHT. 5 



" on whofe conftrudtion the perfection of the inftrument 

 " chiefly depends. But what the refults in theory or by trials 

 " have been, I may poffibly find a more proper occafion to de- 

 *< clare." - 



In the year 1757, the late Mr John Dollond, in confe- 

 quence of fome ftrictures on Sir Isaac Newton from abroad, 

 repeated the noted experiment of refracting a ray of light 

 through prifms of glafs and water, placed with their refracting 

 angles in oppofite directions, and fo proportioned to each other, 

 that the ray, after thefe oppofite refractions, emerged parallel to 

 the incident ray. According to the Newtonian doctrine, there 

 ought here to have been no divergency of the heterogeneal rays, 

 and no colour produced by thefe equal and oppofite refractions. 

 But this was not the refult of the experiment. The ray was 

 coloured very fenfibly ; and the author of the experiment find- 

 ing that he could, by thefe oppofite refractions, produce colour, 

 notwithstanding the parallelifm of the incident and emergent 

 light, with reafon concluded that he might, by properly pro- 

 portioning the refracting angles of his prifms, effect: an inclina- 

 tion of the refracted to the incident light, without any colour 

 or divergency. The event turned out as he expected. 



Pushing his experiments farther, he difcovered, fome time 

 afterwards, that a colourlefs refraction might be produced by 

 a combination of different kinds of glafs, as well as by a com- 

 bination of glafs and water, which feemed to remove complete- 

 ly the great obftacle to the perfection of the refracting telefcope, 

 difcovered by Sir Isaac Newton. 



As it was found foon afterwards, that the other principal im- 

 perfection which limits the performance of telefcopes, namely, 

 the aberration arifing from the fpherical figures of lenfes, might 

 be corrected by properly proportioning to each other the fphe- 

 ricities of the convex and concave lenfes, of which the com- 

 pound object glafs is compofed ; it was expected by men of 

 fcience, that an increafe of the aperture and power of the in- 

 ftrument. 



