[ 742 ] 
not long without hopes of a remedy : for condder- 
ing, the furfaces of fpherical glaffes admit of great 
variations, tho’ the focal didance be limited, and 
that by thefe variations their aberrations may be 
made more or lefs, almoft at pleafure ; I plainly faw 
the poffibility of making the aberrations of any two 
glaffes equal ; and as in this cafe the refractions of 
the two glades were contrary to each other, their 
aberrations, being equal, would intirely vanifh. 
And thus, at lad, I obtained a perfeCt theory for 
making objecft-gladfes, to the apertures of which I 
could lcarce conceive any limits : for if the practice 
could come up to the theory, they mult certainly 
admit of very extenlive ones, and of courfe bear 
very great magnifying powers. 
But the difficulties attending tlie practice are very 
confiderable. In the firlt place, the focal diftances, 
as well as the particular furfaces, mud be very nicely 
proportioned to the denfities or refracting powers of 
the glades ; which are very apt to vary in the fame 
fort of glafs made at different times. Secondly, the 
centres of the two glades mud be placed truly on the 
common axis of the telefcope, other wife the dedred 
effeCt will be in a great meafure dedroyed. Add to 
thefe, that there are four furfaces to be wrought per- 
fectly fpherical ; and any perfon, but moderately 
praCtifed in optical operations, will allow, that there 
mud be the greated accuracy throughout the whole 
work. 
Notwithdanding fo many difficulties, as I have 
enumerated, I have, after numerous trials, and a 
refolute perfeverance, brought the matter at lad to 
fuch an iduc, that I can condruCt redacting tele- 
fcopes. 
