[ 7+i ] 
To make therefore two fpherical glaffes, that fhall 
refradt the light in contrary directions, it is eafy to 
underhand, that one muft be concave, and the other 
convex ; and as the rays are to converge to a real 
focus, the excefs of refradtion mud evidently be in 
the convex ; and as the convex is to refradt molt, it 
appears from the experiment, that it mud be made 
with crown glafs, and the concave with white flint 
glafs. 
And further, as the refractions of fpherical glaffes 
are in an inverfe ratio of their focal diftances ; it 
follows, that the focal diftances of the two glaffes 
fhould be inverfely as the ratio’s of the fractions of 
the wedges : for being thus proportioned, every ray 
of light, that paffes thro’ this combined glafs, at 
whatever diftance it may pafs from its axe, will con- 
ftantly be refradted, by the difference between two 
contrary refradtions, in the proportion required ; and 
therefore the different refrangibility of the light will 
be intirely removed; 
Having thus got rid of the principal caufe of the 
imperfedtion of refradting telelefcopes, there feemed 
to be nothing more to do, but to go to work upon 
this principle : but I had not made many attempts, 
before I found, that the removal of one impediment' 
had introduced another equally detrimental (the fame 
as I had before found in two glaffes with water be- 
tween them) : for the two glaffes, that were to be 
combined together, were the fegments of very deep 
fpheres ; and therefore the aberrations from the fphe- 
rical furfaces became very confiderable, and greatly 
difturbed the diftindtnefs of the image. Tho’ this 
appeared at firft a very great difficulty, yet I was 
