[ 739 ] 
were required to be fo fhort, in order to make the 
refractions in the required proportions, that they 
muff produce aberrations, or errors, in the image, as 
great, or greater, than thofe from the different re- 
frangibility of light. And therefore, feeing no me- 
thod of getting over that difficulty, I gave up all 
hopes of fucceeding in that way. 
And yet, as thefe experiments clearly proved, that 
different fubffances diverged the light very differently, 
in proportion to the refraction ; I began to fufpeCt, 
that fuch variety might poffibly be found in different 
forts of glafs, efpecially as experience had already 
fhewn, that fome made much better objeCt-glafles, 
in the ufual way, than others : and as no latisfaCtory 
caufe had as yet been affigned for fuch difference, 
there was great reafon to prefume, that it might be 
owing to the different divergency of the light by their 
refractions. 
Wherefore, the next bufinefs to be undertaken, 
was to grind wedges of different kinds of glafs, and 
apply them together, fo that the refractions might 
be made in contrary directions, in order to difcover, 
as in the foregoing experiments, whether the re- 
fraction and divergency of the colours would vanifh 
together. But a confiderable time elapfed before I 
could fet about that work ; for tho’ I was determined 
to try it at my leifure, for fatisfying my own curiofity, 
yet I did not expeCt to meet with a difference fuffi- 
cient to give room for any great improvement of 
telefcopes ; fo that it was not till the latter end of 
the year that I undertook it, when my firft trials 
convinced me, that this bufinefs really deferved my 
utmoff attention and application. 
5 B 2 
Idif- 
