C ] 
from the concave Surface FET^. Call the com. 
mon Sine of Incidence, nh the Sine of Refradioii 
of the leaft refrangible Rays out of a denfe Medium 
into a rarer, mh and, of the moft refrangible, 
then, according to the known and received Laws of 
Refradion and Refledion, the Focal Diftance of the 
moft refrangible Rays, from the Vertex of the 
Speculum, (negleding its Thicknefs, as of little 
or no Moment in the prefent Cafe) will be found 
“7 T-~( And the Quantity of the 
greateft Aberration, occafioned by the different Re- 
frangibility of the moft and leaft refrangible Rays, 
will be to tlxe focal Diftance juft mentioned, 
as {a — to {a^e)m~{-em which 
Quantity, or Error, thus obtained, (to abbreviate the 
Calculation) call g ; and now let it be required to 
form a Lens, if poftible, which, placed at feme given 
Point in the Axis between the Focus of the moft 
refrangible Rays F, and the Vertex of the Spe- 
culum (as li), fhall refrad not only the Rays of the 
moft refrangible Kind tending to the Point T, but 
alfo the Rays of the leaft refrangible Kind tending 
to in fuch a Manner, that both Sorts fhall con- 
cur, after fuch Refradion, in feme other Point of 
the Axis 7? > l^t HB the given Diftance of the Point 
in the Axis 7/, from the Focal Point 7^, be called 
d } and then if the Point H has been aflumed, fo that 
the faid given Quantity, or Diftance, 7, is greater 
than but lefs than i fay the re- 
fjL^m '' 
frading Superficies GHE that fhall perform what 
was required, will be part of a concave Sphere, 
whofe 
