C r 93 3 
the air, may be afiigned, as fhall unite the extreme, 
and all the intermediate, rays in the fame focus F ; 
neglecting the aberration from the figure. 
The RULE is this i 
For the diflances of the points of divergence from 
the lens, write D the greater, and d the leaft ; the 
femidiameter of any of the given furfaces being af- 
fumed for unity: And — , — exprefiing the ratios of 
the fines of incidence and refraCtion of the violet and 
red rays out of air into the laft medium whofe fur- 
face is required : the femidiameter of that furface 
w iU be 7? as ma y be ea % demonftrated 
from a theorem of Dr. Smith, in the remarks fubjoin- 
ed to his Optics. 
Thus if the laft medium is glafs, the femidiame- 
ter of the furface from which the rays pafs into the 
air, muft be -g D _ 77 ^» it being, in this cafe, — =t^» 
m 
Example I. 
Let M/> CN cM (Fig. 8.) be a double convex 
lens of water confined between the plano-concave 
MTLN and the menifcus MKNcM, both of glafs, 
and having the radii of their furfaces contiguous to 
the water, equal to each other, or to unity . an 1 
a ray S p, parallel to the common axis of the lenies, 
after being refrafted by the aqueous lens, have its 
Vol. LIII. Dd extreme 
