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

In order to underſtand well how Mathema-
ticks may be apply’d to Drawing; let us ſup-
poſe a Man A, viewing an Object; and be-
tween him and the Object he looks at, let us
imagine a tranſparent Plane C. Suppoſe more-
over, that Lines be drawn upon this Plane, as
in D, which cover the Bounds of the Object B
in reſpect of the Spectator A, and each Part that
he ſees thereof. Now, ſince all Objects are ſeen
by the Rays of Light coming from every of
their Points, and terminating at the Eye, and
not otherwiſe; and ſince that here all the Rays
proceeding from the Object B, likewiſe paſs
thro’ every Point of the Repreſentation D; it is
manifeſt, that this Repreſentation will have the
ſame Effect upon the Spectator’s Eye, as the
ſaid Object B hath. Now, by means of Geo-
metry, we can find the Points of the Figure D,
on the Plane C, placed in a given Situation,
thro’ which the Rays coming from the Object B
to the Eye of the Spectator A, do paſs; and
theſe Points are the Interſections of the Rays
and the Plane. Alſo, (as others have very well
obſerv’d) a Perſpective Plane, or Picture in Paint-
ing, may be conceiv’d as a Window, upon which
the Objects ſeen thro’ it are repreſented.

### 8.1.

Fig. 1.

Now, without Mathematicks, this Repreſen-
tation cannot be well found: For when Objects
are drawn by only viewing, or looking at them; their true Repreſentations after this way, will
be very often miſs’d on; whereas, by Geome-
try, we can always obtain them.

This Obſervation only, is ſufficient to eſta-
bliſh the Neceſſity of Perſpective: Tho’ there are
ſome Painters, who (according to the common
Maxim) affirm, That what they do not know
of this Art, is not worth the Pains of learn-
ing.

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