DOI Page / Citation link:
https://doi.org/10.11588/diglit.18826#0160
LECT. III.] ON PERSPECTIVE.
113
PLATE XLI.
No. I. Shews how to find the fhadow of a globe
enlightened by the fun: here we may obferve, that
the fhadow of a globe is fimilar to that of a circle
directly oppofed to the luminary ; by finding there-
fore the fhadows of certain points in its circumfe-
rence fo oppofed, we obtain the whole. R is a ray
of light; V the center of the picture; 1 2 3 4 is a
fuppofed fection, defcribing the enlightened part:
procure the feat of this circle on the ground, by per-
pendiculars, as 1 a, 2 b, 3 c, 4 d; rule lines parallel
to R, from 1 2 3 4 for the fhadow; and lines parallel
to the ground plane from abed: their intersections
afcertain the feats of the fhadows of thofe points
(1 2 3 4) in the original fection; which, being
joined, conduct the reft of the fhadow of the circum-
ference.
No. II. Is a globe enlightened by a lamp: now
as the luminary is fo near to the object, a much lefs
portion than half the circumference is alone capable
of receiving light. We have to fuppofe a fimilar
fection as before; 1 2 3 4; find their feats on the
ground, by perpendiculars, as abed; rule L 1,
L,2, L3, L4; and from the feat of the light S, rule
interferons, as L 1 in e; L 2 in/; L 3 in g ; L 4
in h. Thefe points, united, indicate the fhadow of
the original fection. Y is the center of the picture,
PLATE
113
PLATE XLI.
No. I. Shews how to find the fhadow of a globe
enlightened by the fun: here we may obferve, that
the fhadow of a globe is fimilar to that of a circle
directly oppofed to the luminary ; by finding there-
fore the fhadows of certain points in its circumfe-
rence fo oppofed, we obtain the whole. R is a ray
of light; V the center of the picture; 1 2 3 4 is a
fuppofed fection, defcribing the enlightened part:
procure the feat of this circle on the ground, by per-
pendiculars, as 1 a, 2 b, 3 c, 4 d; rule lines parallel
to R, from 1 2 3 4 for the fhadow; and lines parallel
to the ground plane from abed: their intersections
afcertain the feats of the fhadows of thofe points
(1 2 3 4) in the original fection; which, being
joined, conduct the reft of the fhadow of the circum-
ference.
No. II. Is a globe enlightened by a lamp: now
as the luminary is fo near to the object, a much lefs
portion than half the circumference is alone capable
of receiving light. We have to fuppofe a fimilar
fection as before; 1 2 3 4; find their feats on the
ground, by perpendiculars, as abed; rule L 1,
L,2, L3, L4; and from the feat of the light S, rule
interferons, as L 1 in e; L 2 in/; L 3 in g ; L 4
in h. Thefe points, united, indicate the fhadow of
the original fection. Y is the center of the picture,
PLATE