0

P T I C S.

j)C e

&

(V

st-'

'i

0

brf

P'>s

st,

■r

; st

0

0I ¥

r *

st

st

st

0

st

V

-st'

st,

st 1

JP

st’

^stance from the Image. By this MeansRays coming from the Images are re-b flected

• ire %, and as the focal Distances of the Eye-Glasses'tersely. Let these be represented by F and F, and, /> in any two Telescopes ; let D, d, be the Diame-

rs °f the Apertures, and L, /, the linear Dimensionsr-

° fth e Pict UrHe

f /'

ares; then we have D : d :: L

"L ° H tt r “ r'-

H I, as f J4 .) - (bec»ft L = J

J the last) therefore if D or f be each increased in anys£ tl0 > the Distinctness will remain the same as before,J,f f ' 49-) and the linear Dimensions of the Imageas n l dim in>slrcd in the fame Ratio, (since L is inverselyi f [ b ut the Brightness of the Image will be ^creased0 quadruplicate Ratio of what it had besom I o ,oivl 7 ' S "PP 0 se F or the focal Length of th .e Telefc pL. is 'b then the Brightness of the Picture will be m thisp e as 1L L - and if D and f he increased each in the(3 10 of i to’ m, then will the Brightness be in thisn e us in" si)- st rd- — D 1 f 1 ; so that the former.* n Shtnes s i s to this as D 1 st toD* st m\ that is, as Ito If * w hich Ratio is quadruplicate of the Ratio I

s8 - Because we had D : ^, or D f: F, when Objects

£ ear e stua!ly bright, (by Art. ZZ-) and when they areth e 7r squally distinct we had D : f (hy 49,)*tW 6 Ai e in refracting Telescopes of various Lengths,dist- 9H ects may appear equally bright and equaliy‘nct 5 i t j s requisite t si at si) 1 ; F, and f 1 : F, or that

(ilr f ; i that is, the Diameter of the Aperture and

the l 3e f ocni l en gth of the Eye-Glass should each he a:

f fmre Root of the focal Distance er Length of the* ie pope % .

F 3 59- It4