OPT 
Incidence to the fine of its angle of refraction ; and fo 
is radius, or ioooooo, to 999736 ; which, therefore, is 
the proportion between the fine of incidence, in vacuo and 
the fine of refraction from thence into common air. 
Grimaldi firft obferved, that the coloured image of the 
fun refraCted through a prifm is always oblong, and that 
colours proceed from refraCtion. The way in which he 
firfi;.difcovered this was by Vitellio’s experiment, in which 
a piece of white paper placed at the bottom of a glafs 
veflel filled with water, and expofed to the light of the 
fun, appears coloured. But he obferved, that, in cafe 
the two furfaces of the refraCted medium were exaCtly 
parallel to each other, no colours were produced. Of 
the true caufe of thofe colours, however, he had not the 
lead: fufpicion. This difcovery was referved for fir Ifaac 
Newton. 
Having procured a triangular glafs prifm to fatisfy him- 
felf concerning the phenomena of colours, this great 
man was furprifed at the oblong figure of the coloured 
fpeftrum, and the great difproportion betwixt its length 
and breadth ; the former being about five times the mea- 
fure of the latter. After various conjectures refpeCting 
the caufes of thefe appearances, he fufpeCted that the co¬ 
lours might arife from the light being dilated by fome un- 
evennefs in the glafs, or fome other accidental irregula¬ 
rity ; and to try this, be took another prifm like the for¬ 
mer, and placed it in fuch a manner, that the light, paf- 
fing through them both, might be refraCted in oppofite 
directions, and thus be returned by the latter into the 
famecourfe from which it had been diverted by the for¬ 
mer. In this manner he thought that the regular efreCts 
of the firfi: prifm would be augmented by the multiplicity 
of refraCtions. The event was, that the light, diffufed 
by the firft prifm into an oblong form, was by the fecond 
reduced into a circular one, with as much regularity as 
if it had not paded through either of them. He then hit 
upon what he calls the experimentam crucis; arid found 
that light is not fimilar, or homogeneous ; but that it 
confifts of rays, fome of which are more refrangible than 
others; fo that, without any difference in their incidence 
on the fame medium, fome of them fhall be more refrac¬ 
ted than others; and therefore, that, according to their 
particular degrees of refrangibility, they will be tranfmit- 
ted through the prifm to different parts of the oppofite 
wall. Since it appears, from thefe experiments, that dif¬ 
ferent rays of light have different degrees of refrangibi¬ 
lity, it follows, that the rules laid down by preceding 
philofophers concerning the refraCtive power of water, 
glafs, &c. muff be limited to the mean rays of the fpec- 
trum. Sir Ifaac, however, proves, both geometrically 
and by experiment, that the fine of the incidence of every 
kind of light, conlidered apart, is to its fine of refraCtion 
in a given ratio. 
The moll important difcovery concerning refraCfion 
fince the time of fir Ifaac Newton, is that of Mr. Dollond, 
who found out a method of remedying the defeCts of re- 
fraCting telefcopes arifingfrom the different refrangibility 
of light. Sir Ifaac Newton imagined that the different 
rays were refraCted in the fame proportion by every me¬ 
dium, fo that the refrangibility of the extreme rays 
might be determined, if that of the mean ones were given. 
From this it followed, as Mr. Dollond obferves, that equal 
and contrary refractions muft not only deltroy each other, 
but that the divergency of the colours from one refrac¬ 
tion would likewile be corrected by the other, and that 
there, could be no poflibility of producing any fuch thing 
as refraftion without colour. Hence it was natural to 
infer, that all objeCt-glafles of telefcopes muft be equally 
affeCted by the different refrangibility of light, in pro¬ 
portion to their apertures, of whatever materials they may 7 
be formed. 
For this reafon, philofophers defpaired of bringing re¬ 
fracting telefcopes to perfection. They therefore applied 
themfelves chiefly to the improvement of the reflecting 
telefcope till 1747; when M. Euler, improving upon a 
VOL. XVII. No. 1197. 
ICS. 549 
hint of fir Ifaac Newton’s, propofed to make objecl-glafTes 
of water and glafs; hoping that, by their difference of re¬ 
fraCtive powers, the refractions would balance one. ano¬ 
ther, and thereby prevent the difperfton of the rays that 
is occafioned by their difference of refrangibility. This 
memoir of M. Euler excited the.attention of Mr. Dollond. 
He went over all M. Euler’s calculations, fubftituting 
for his hypothetical laws of refraCtion thofe which had 
been afeertained by Newton ; and found, that it followed 
from Euler’s own principles, that there could be no union 
of the foci of all kinds of colours but in a lens infinitely 
large. 
Euler did not mean to controvert the experiments of 
Newton ; butafferted, that, if they were admitted in all 
their extent, it would be impoffible to correCt the differ¬ 
ence of refrangibility occafioned by the tranfmiffion of the 
rays from one medium into another of different denfity; 
a correction which he thought was very poffible, fince he 
l'uppofed it to be effected in the eye, which he confidered 
as an achromatic inffrument. To this reafoni ng Mr. Dol¬ 
lond made no reply, but by appealing to the experiments 
of Newton, and the circumfpeCtion with which it was 
known that he conducted all iiis enquiries. 
This paper of Euler’s was particularly noticed by M. 
Klingenftierna of Sweden, who found that, from New¬ 
ton’s own principles, the refult of his 8th experiment 
could not anfwer his defeription of it. Newton found, 
that when light-paffes out of air through feveral media, 
and thence goes out again into air, whether the refraCt in 
furfaces be parallel or inclined to one another, this light, 
as often as by contrary refraCtions it is fo corrected as to 
emerge in lines parallel to thofe in which it was incident, 
continues everafter to be white; but, if the emergent rays 
be inclined to the incident, the whitenefs of the emerging 
light will, by degrees, become tinged at its edges with 
colours. This he tried by refraCting light with prifms of 
glafs placed within a prifmatic veil'd of water. By theo¬ 
rems deduced from this experiment he infers, that the 
refraCtions of the rays of every fort, made out of any me¬ 
dium into air, are known by having the refraCtion of the 
rays of any one fort; and alfo that the refraCtion out of 
one medium into another, is found as often as we have 
the refraCtions out of them both into any third medium. 
On the contrary, the Swedi/li philofopher obferves, that, 
in this experiment, the rays of light, after palling through 
the water and the glafs, though they come out parallel to 
the incident rays, will be coloured ; but that, the fin all er 
the glafs prilin is, the nearer will the refult of it approach 
to Newton’s defeription. 
This paper of M. Klingenftierna, being communicated 
to Dollond, made him entertain doubts concerning New¬ 
ton’s report, and induced him to have recourfe to expe¬ 
riment. He therefore cemented together two plates of 
glafs at their edges, fo as to form a prifmatic velfel when 
flopped at the ends; and, the edge being turned down¬ 
wards, he placed in it a glafs prifm with one of its edges 
upwards, and filled up the vacancy with clear water, 
fo that the refraCtion of the prilin was contrary to that 
of the water, in order that a ray of light, tranfmitted 
through both thefe refraCting media, might be affected 
by the difference only between the two refraCtions. As 
he found the water to refraCt more or lefs than the glafs 
prifin, he diminiflied or increafed the angle between the 
glafs plates, till he found the two contrary refraCtions 
to be equal ; which he difeovered by viewing an objeCt 
through this double prifm : for, when it appeared neither 
raifed nor deprefled, he was fatisfied that the refraCtions 
were equal, and that the emergent and incident rays were 
parallel. 
But, according to the prevailing opinion, the objeCl 
lliould have appeared of its natural colour; for, if the 
difference of refrangibility had been equal in the two 
equal refraCtions, they would have rectified each other. 
This experiment, therefore, fully proved the fallacy of 
the received opinion, by fhowing the divergency of the 
7 A light 
