REFRANGIBILITY of LIGHT. F 
“ on whofe conftruétion the perfection of the inftrument 
“* chiefly depends. But what the refults in theory or by trials 
“ have been, I may poflibly find a more proper occafion to de- 
“* clare.” 
In the year 1757, the late Mr Joun DoLtonD, 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 refraGtions, emerged parallel to 
the incident ray. According to the Newtonian do@trine, there 
ought here to have been no divergency of the heterogeneal rays, 
and no colour produced by thefe equal and oppofite refractions. 
Bur 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 refrations, produce colour, 
— notwithftanding the parallelifm of the incident and emergent 
light, with reafon concluded that he might, by properly pro- 
portioning the refracting angles of his prifms, effeét an inclina- 
tion of the refracted to the incident light, without any colour 
or divergency. The event turned out as he expected. 
Pusuinc 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 NewTown. 
As it was found foon afterwards, that the other principal im- 
perfeGtion which limits the performance of telefcopes, namely, 
the aberration arifing from the {pherical figures of lenfes, might 
be correGted by properly proportioning to each other the {phe- 
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, 
