Optics.

flected back through the central Hole C $

of the large Mirrour, where they fall ofl

' • . ' ' ' ‘

59. In this £ase likewise the linear Dimensions of^f

P'ttture ar Image are in the fame fubduplkate R.atio of ',Length of the Telescope \ because, as was-shewn, (Art. \the linear Dimensions are directly as the Diameter ofAperture, which is here strewn to be as the Square R°°of the Length of the Telescope. .

60. In reflecting Telescopes, when the Distinctly

is given, we have F : ~-, and therefore^ 3 : D 2 F.Article 51.) Also when the Brightness is given we have/'

^ , (Art. 55. ) therefore F : —. Hence, when the

f v ' y 3 .

tinctnefs and Brightness are both given, we have 'j

IT

(D 1 F)

ory 4 : D 3 , ory : D

r*

61.

D

The linear Dimensions of the Picture — v ? c{6

F

as yy that is, in this Cafe, ^ : D,, and therefore $’

.... x

FD|; whence F : ff. : D

jjr : D 2 . Hence in resetting

scopes of different Lengths a given Objett will apffL■equally distinct and bright , when the Diameters of the-™ej ect-Metals are as the Biquadrate Roots of the Cubes vthe Diameters of the Spheres, or focal Lengths of the Sf etula ; or, 'when the focal Distances of the Eye-Glajfes #] ■as the Biquadrate Rooi of the focal Distance of the Spe& .

62. According to the. Theorems in Art. 48, 49,genius calculated a Table of the linear Aperture of £ \Object-plass, the focal Distance of the Eye-Glass,the linear Amplification or magnifying Power of £ ^Telescope from one which he found by Experienceconstructed in the best Manner. I have reducedRhinland Measures to English Feet, Inches, and V cC> 'thai Parts, as follows, ' j

: ■ > ■ ' flCS: