8thly. As the image is formed only by the reflected,
rays of the fun, fo due care mould be taken that none of
the fun's direct rays fall on the lens in the window ; for
if they do, they will, by mixing with the former, greatly
difturb the picture, and render it very confufed and un-
pleafant to view.
9thly. As white bodies reflect the incident rays mofl
copioufly, and black ones abforb them moft ; fo to make
the picture moft perfect it ought to be received upon a
very white furface, as paper, a painted cloth, wall, &C.
bordered round with black, that fo the collateral rays
which come from on each fide the object may be ftifled,
and not fufFered to difturb the picture by reflection.
Thefe are the neceffary precautions for the due or-
dering of the various circumftances of this experiment.
We fhall enumerate the feveral principal phenomena of
the dark chamber. The firft of which is, that an exact
and every way fimilar image is formed of an external
object; for pencils of rays coming from all points of the
object will reprefent thcfe points in fuch a manner and
pofition as will be proportional and correfpondent to
their refpective pofitions and diftances in the object, fo
that the whole in the image fhall bear an exact fimili-
tude or likenefs of the object in every refpect.
The fecondphenomenon is, that the image will bear the
fame proportion to the object, whether aline, fuperhcies,
or folid, as their diftances from the glafs reflectively.
Hence the larger the focal diftance from the glafs, the
more ample will be the picture of the fame object, but