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I difcovered a difference, far beyond my hope?, 
in the refractive qualities of different kinds of glafs, 
with refpeCt to their divergency of colours. The 
yellow or ftraw- coloured foreign fort, commonly 
called Venice glafs, and the Englifh crown glafs, are 
very near alike in that refpeCt, tho’ in general the 
crown glafs feems to diverge the light rather the 
lead; of the two. The common plate glafs made in 
England diverges more ; and the white cryffal or 
flint Englifh glafs, as it is called, mod; of all. 
It was not now rny bufinefs to examine into the 
particular qualities of every kind of glafs that I could 
come at, much lefs to amufe myfelf with conjectures 
about the caufe, but to fix upon fuch two forts as 
their difference was the greateff ; which I foon found' 
to be the crown, and the white flint or cryftal. I' 
therefore ground a wedge of white flint of about 2 y 
degrees, and another of crown of about 25) degrees, 
which refraCted nearly alike j but their divergency of 
the colours was very different. I then ground feve- 
ral others of crown to different angles, till I got 1 
one, which was equal, with refpeCt to the divergency 
of the light, to that in the white flint: for when 
they were put together, fo as to refraCt in contrary 
directions, the refraCted light was intirely free from 
colour. Then meafuring the reflations of each 
wedge, I found that of the white glafs to be to that 
of the crown nearly as 2 to 3 ; and this proportion 
would hold very nearly in all fmall angles. Where- 
fore any two wedges made in this proportion, and 
applied together, fo as to refraCt in a contrary di- 
rection, would refraCt the light without any diffe- 
rence of refrangibilitv. 
To 
