46 O ? t i c Si-

lts Image form’d at a greater Distance <$■the other Side, and proportionably large,

PlateXLV.Fig. 8.

is contain’d between the Rays D d and Fy parall eto the Axis ; that is, the visible Part of the Image 15df— D F, and is seen under the Angle DLF (I>hging the Focus of the Lens D F, or LE = CE) atf®this is call’d the Optic Angle , or Angle of visiblenitude.

13. But this Angle is greatly increased by the A"'dition of the Glass GK; for let a K and b G be th®Axes of two Pencils of Rays which come from thetreme Parts of the Object a and b, and falling divets'ing upon the Lens G K are refracted by it towardsPoint O in the Axis of the Glass produced. But iftheir Passage they are intercepted by the Lens D F’which refracts them to a Point I nearer the Lens th^the Focus L ; and therefore the Angle D I F is greatsthan the Angle DLF, and consequently a larger Fid®of View is by this Means obtain’d.

14. To find in what Proportion the Angle D I Fgreater than DLF, we must consider e as a RadialiPoint, whence the Pencil of Rays G e K divergesthe Lens G K ; then if H e = d, H a — r Radiusthe double and equally convex Lens G K, we hA e

dr ’(

—- —y — DO the Focus after Refraction ; and >'

a — r

H E = f>, then y — p — EO — d — Distance for coss.verging Rays K F and G D. And let r — RadMof the double and equally convex Lens D F;

= f- EI = Wherefore LE : IE :: * J

d -f r y — p X r

dr ,

y_iArj ±.. j. y-p _ d—_r p . Tha do( s

y —£-f-r y — p-\- r dr

- p + r

d — r

when the several Quantities d,r,p, r, are given, ^

4 ' Rs^