35
Optics.
is ,not distinct, unless by Rays which
are parallel, or nearly so ; for those only
will
require a greater focal Distance to be united in the Axis*and consequently the Focus may be made to fall verynicely on the Retina, by using a Lens E F of a properDegree of Concavity ; and therefore distinct Vision wistbe effected in the fame Manner as in an Eye of a just;
Conformation, by painting the Image on the Retina.
26. Since the Point C is nearer to the Eye than thePoint C, the apparent Place of Objects seen through aconcave Lens is nearer than the true Place y or the Oo-ject will appear at 0 B, instead of O B. And also sinceconverging Rays O a, B b t proceed less converging af-ter Refraction than before, the Object appears under aLss Angle, and therefore the apparent Magnitude orObjects seen by a concave Lens is less than the true. ,
27. The Object is less luminous or bright seen throsuch a Lens than without it j because the Rays beingre ndered more divergent, a less Quantity enters the Pupilof the Eye than otherwise would do. But the Pictureis always more or less bright or enlightened, accordingas it is made by a greater or less Quantity of Rays.
28. Lastly, it appears from what has been said, thatVben a concave Lens E F cannot be applied, we maystill effect distinct Vision by lessening the Distance be-tween the Object and the Eye ; for it is plain, if O Bbe situated at O B, the Image at IM will recede to i tn plateu P°n the Retina, and be distinct in the fame Manner as
^hen made so by the Lens E F. pjg. 5,
29. On the other hand, when the Cornea or Crystal-ib> e is too fiat, (as often happens by Age) an Object’ 1 p' ace( i at the fame Distance from the Eye P C as
before, will have the Rays C 0, C 0 after Refraction in’ the Eye proceed to a Focus beyond the Bottom of the
Eye, in which if a Hole were made (in an Eye taken^ ut of the Head) the Rays would actually go on, and0rm the Image i m ; which Image must therefore b- r - r - J -- 1 ^
the image i m ; wmcn''t^y confused and indistinct on the Retina.
D L
30.
T*