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experiment, a perfeft knowledge of its direc-
tion.
Whoever will interpose an opaque body
between the origin and source of light, natural
or artificial, and any proper substance exposed
to its rays, will easily perceive their illumina-
tions are precluded from the objest furthest
off, by their linking againll that which is
near est; and also, that a diredt line from any
part of the space where light is suspended to
the luminary, will pass through a correspond-
ent part of that objedl which suspends the light.
For instance, when I hold my hand between
the candle and the v/ainscot, it prevents the
rays of light from palling further, and, con-
sequently, occasions a blank of light on the
wainscot; which blank is diredtly strait from
the candle. And so very minute and corres-
pondent are the rays of light to the form of
whatever impedes their progress, that they as-
sume exa&ly its figure and outline, and de-
scribe one perfectly similar, on the nearest su-
perficies which may receive it,
Considered as related to perspedlive, light
divides into two kinds : to each of them
attention is necessary. First, the natural
solar, or lunar light, whose origin being im-
mensely distant from us, and beyond all pro-
portion with respeft to obje&s illuminated
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