REFRANGIBILITY of LIGHT. 45 
with glafs.° Hence I was led to conclude, that if I took an 
achromatic convex lens, compofed of the two effential oils, and 
combined it with an achromatic concave lens of a longer focal 
diftance, compofed of crown-glafs and either of the effential 
oils, I fhould be able, through fuch a double compound object- 
glafs, to converge the rays to a focus, without any aberration 
whatever fromthe difference of refrangibility of light. For if 
the compound convex and compound concave are properly pro- 
portioned to each other, the fecondary fpe@rums, or fringes of 
green and purple, may be rendered of the fame breadth in both 
lenfes ; and from the obférvations before related, this will hap- 
pen when there is a confiderable balance of refraction in favour 
of the convex lens. For it is compofed -of materials which 
form a much narrower fecondary fpeGtrum, under an equal re- 
fraGtion of the whole pencil, than thofe mediums do, of which 
the compound concave is formed». 
Tuis will be anderftood, by attending to what takes place 
in the refraétions of light through the Jenfes, without again re- 
i to are more o halite cafe of prifins. 
Fie. 17% YW peite a cbittorntt concave lens, formed of a 
concave lens of glafs, and a concave lens of a difpetfive fluid, 
but. of a fhorter focus than the concave lens, and fo proportion- 
ed ag to produce a tefraGtion as free from colour as can be ob- 
tained by a “combination of thefe two mediums. This lens 
being expofed to parallel Tays, will make them diverge, after re- 
fraction, from its virtual focus, and the united red and violet 
rays will be the leaft refracted, and will be inclined in a certain 
angle to the green rays which are moft refracted, as reprefented 
- inthe figure. of svi P 
Fic. 18. reprefents a compound convex lens, formed of 
a convex of an effential oil, which difperfes the rays in 
a a Teffer’ degree, combined with a’ concave of an effential oil, 
which 
