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increafe, the divergency of the coloured rays would 
have been greatly diminifhed, or intirely redified ; 
and there would have been a very great refradion 
without colour, as now I had a great difcolouring 
without refradion : but the inconveniency of fo 
large an angle, as that of the veffel muft have been, 
to bring the light to an equal divergency with that 
of the glafs prifm, whofe angle was about 60 de- 
grees, made it neceffary to try fome experiments of 
the fame kind, by fmaller angles. 
I ground a wedge of common plate glafs to an 
angle of fomewhat lefs than 9 degrees, which re- 
fraded the mean rays about 5 degrees. I then made 
a wedge-like veffel, as in the former experiment, 
and filling it with water, managed it fo, that it re- 
fraded equally with the glafs wedge ; or, in other 
words, the difference of their refradions was no- 
thing, and objeds viewed thro’ them appeared nei- 
ther raifed nor depreffed. This was done with an 
intent to obferve the fame thing over again in thefe 
fmall angles, which I had feen in the prifm : and it 
appeared indeed the fame in proportion, or as near as 
I could judge ; for notwithftanding the refradions 
were here alfo equal, yet the divergency of the co- 
lours by the glafs was vaftly greater than that by the 
water ; for objeds feen by thefe two refradions were 
very much difcoloured. Now this was a demon - 
ftration, that the divergency of the light, by the 
different refrangibility, was far from being equal in 
thefe two refradions. I alfo faw, from the pofition 
of the colours, that the excefs of divergency was in 
the glafs j fo that I increafed the angle of the water- 
wedge, by different trials, till the divergency of the 
Vol. 50. 5 B light 
