C *94 ] 
extreme rays, the red and violet, divergent from the 
points D and d ; the diftance of F, the focus where 
all the rays can meet, will be 8.898 : and when this 
happens, the exterior furface oi the menifeus, that 
is, the furface reprel'ented by MPKN, will have its 
radius to that of the inner iuriace McN, as 139 
to 154. 
Example II. 
4 
When a double concave of glafs (the radii of whole 
furfaces are unity) is inclofed in water, as in Fig. 9, the 
water being confined on one fide by a thin glafs plate 
TL, and on the other by a concentrick lpherical fhell 
MPKN i the femidiameter of this fhell muff; be to 
unity as 47 1 to 547 : and the focal diftance CF, at 
which the colourlefs image is formed, will be 4.774. 
In thefe examples the thicknefs of the lenfes is neg- 
lected ; but it may eafily be taken into the account, if 
it is thought neceffary. 
The fame thing may be effected by means of any 
media of different refraCtive powers : for the femidi- 
ameter of the laft refraCting furface being determined 
according to the foregoing rule, the nearer diftance of 
the points of divergence ( d ) of the more refrangible 
rays will be fo compenfated by their greater refrangibi- 
lity, that all the rays will converge to the fame focus F. 
And this without introducing any new principle into 
the fcience of optics, or any difperfion of light diffe- 
rent from the refractions difeovered by Sir Ilaac New- 
ton near a hundred years ago. 
XXXII. An 
