## Full text: Gravesande, Willem Jacob: An essay on perspective

An ESSAY on out the Camera Obſcura, is ſimilar to that which
they form, after having paſſed through the Glaſs: Therefore the Rays which fall upon the Paper in
the Camera Obſcura, likewiſe give the true Re-
preſentation of the Objects thereon. Which was
to be demouſtrated.

Theſe Objects appear inverted, becauſe the
Rays croſs each other in paſſing through the Glaſs; thoſe coming from above going below, & c.

## 199. Theorem II.

3. The Reflection which the Rays of Light ſuffer
upon a plain Mirrour or Speculum, before they fall
upon a Convex Glaſs, no-wiſe deforms the Repreſen-
tation of Objects.

This is evident: For the Speculum reflects the
Rays in the ſame Order as it receives them.

Now to ſhew the Uſe that may be drawn from
the Camera Obſcura in Deſigning, I ſhall here lay
down the Deſcription and Uſe of Two Machines,
which I uſe for this End.

## 200.The Deſcription of the Firſt Machine.

4. This Machine is ſomething in Figure of a
Chair, (ſuch as People are carried in) the back Part
of the Top is rounded, and its Foreſide P Q ſwells
out in the middle: Vide Fig. 70. which repreſents
the Machine, the Side whereof oppoſite to the
Door, is ſuppoſed to be raiſed up, that ſo its In-
ſide may be ſeen.

### 200.1.

Fig. 70.

5. The Board A within-ſide, ſerves as a Ta-
ble, and turns upon two Iron-pins, going into
the Wood of the Fore-ſide of the Machine, and is
ſuſtain’d by two ſmall Chains, that ſo the ſaid
Table may be lifted up; and therefore one may

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