[ S6 ] 
in thofe obie£t glafles, when concave eye glafTes were 
ui'ed, and thefe have been ever fince made in this 
manner: fome trials were likewife made, at the fame 
time, to enlarge the apertures of longer objedt glaffes, 
where convex eye glaffes were ufed, by the fame 
method ; but thefe not fucceeding, in the fame man- 
ner, the method of making them with one lens of 
crown glafs, and one of white flint glafs, -was con- 
tinued. 
As I could not fee any good reafon why the me- 
thod, which was pradiced with fo much fuccefs, 
when concave eye glaffes were ufed, fliould not do 
with convex ones ; 1 determined to try fome further 
experiments in that way. After a few trials, I found 
it might be done; and in a fhort time I finilhed an 
objed glafs of 5 feet focal length, with an apperture 
of 3 ^ inches, compofed of two convex lenfes of 
crown glafs, and one concave of white flint glafs. 
Thinking that the apertures might be yet admit- 
ted larger ; I attempted to make one of 3 4. feet fo- 
cal length, with the fame aperture of 3 4 inches, 
which 1 have now completed, and am ready to ffew 
the fame to the Royal Society, if defired. 
The difficulty of procuring good glafs of fo large 
a diameter, and of the thicknefs required, added to 
the great exadnefs of the furfaces, in order to cor- 
red the aberration in fuch large apertures, has pre- 
vented me from attempting to extend them any farther 
in that length. I am, 
S I R, 
Your moft obedient, 
and moft humble fervant, 
Peter Dollond. 
VIII. Some 
