[ 55 ] 
S 1 R, 
Read Feb. 7, 
1765. 
I Take the liberty of fending you the 
following fliort account of an improve- 
ment I have lately made in the compound objedt 
glafles of refradting telefcopes. 
The diffipation of the rays of light may be per- 
fedlly cor reded in objedt glaffes, by combining me- 
diums of different refradlive qualities ; and the errors 
or aberrations of the fpherical furfaces may be cor- 
redted by the contrary refradlions of two lenles, made 
of the different mediums; yet as the excefs of re- 
fradtion is in the convex lens, and though the fur- 
faces of the concave lens may be fo proportioned as 
to aberrate exadUy equal to the convex lens, near the 
axis i yet as the refradlions of the two lenles are not 
equal, the equality of the aberrations cannot be con- 
tinued to any great didance from the axis. 
In the year 1758, when my father had condrudled 
fome objedl glaffes for telefcopes in this manner, viz. 
with one convex lens of crown glafs, and one con- 
cave lens of white flint glafs ; he attempted to make 
fliort objedl glaffes to be ufed with concave e)^ glaffes; 
in the fame manner ; but as the field of view, in 
ufing a concave eye glafs, depends on the aperture 
of the objedl glafs, the limits of the aperture were 
found to be too fmall : this led my father to confider 
that if the refradtion of the crown glafs (in which 
the excefs was) fhould be divided by means of having 
two lenfes made of crown glafs inflead of one, the 
aberration would thereby be decreafed, and the aper- 
tures might then be larger; this was tried with, fuccds 
j in 
