on PERSPECTIVE.
ved backwards or forwards, or elſe the Looking-

glaſs raiſed or lower’d, until the Rays proceed-

ing from the Statue may be reflected by the Mir-

rour upon the Convex Glaſs. When theſe Alte-

rations of the Box, or Mirrour, are not ſufficient to

throw the Rays upon the Convex Glafs, the whole

Machine muſt be removed backwards or for-

wards.

##
203.
Demonstration
.

Concerning the before-mention’d Inclination of the

Mirrours.

19. In order to demonſtrate, that the Mirrour

L hath been conveniently inclin’d, we need on-

ly prove, that the reflected Rays fall upon the

Table A under the ſame Angle, as the direct

Rays do upon a Plane, having the ſame Situation

as one would give to the Picture.

Now let A B be a Ray falling from a Point of

ſome Object upon the Mirrour G H, and from

thence is reflected in the Point a upon the Table

of the Machine: We are to demonſtrate, that if

the Line D I be drawn, making an Angle with

FE equal to the Inclination of the Picture; that

is, if the Angle DIE be the double of the Angle
D F I; I ſay, we are to demonſtrate, that the

Angle B a f is equal to the Angle BCD.

The Angle DIE, by Conſtruction, is the double

of the Angle DFI; and conſequently this laſt Angle

is equal to the Angle I D F; and ſince the Angle

of Incidence C B D is equal to the Angle of Re-

flection a B F, the Triangle BCD is ſimilar to

the Triangle F a B: Whence it follows, that the

Angle Ba F is equal to the Angle BCD. Which

was to be demonſtrated.