C A M 
it towards the place of re-prefentation; and, to prevent 
■any rays from palling afide it, let the picture be encom- 
•palfcd with fome board or cloth. If the objedl be a llatue, 
-or a living creature, it mult be much enlightened by call¬ 
ing the fun’s rays on it, either by reflection, refradtion, or 
both. Between this object and the place of reprefenta- 
tion put a broad convex glafs, ground to Inch a convexity 
as that it may reprefent the objedt diftindtly in Inch place. 
The nearer this is fituated to the object, the more will the 
image be magnified on the wall, and the further the lefs ; 
fitch diverfity depending on the difference of the fpheres 
of the glaffes. If the objedt cannot be conveniently in¬ 
verted, there mufl be two large glaffes of proper fpheres, 
fituated at fuitable diftances, eafily found by trial, to make 
the reprefentations eredt. This whole apparatus of ob¬ 
ject, glaffes, &c. with the perfons employed in the ma¬ 
nagement of them, are to be placed without the window 
or hole, fo that they may not be perceived by the fpedta- 
tors in the room, and the operation itfelF will be eafily per¬ 
formed. Phil. Tranf. Ncr. xxxviii. p. 741. 
CA'MERA OBSCU'RA, or Dark Chamber, an op¬ 
tical apparatus, contrived by means of the fcioptic ball, 
or ball and focket, containing a lens, representing an arti¬ 
ficial eye, by which the images of external objects are 
fhewn diftindtly, and in their native colours, on a white 
ground placed within the room, in the focus of the lens. 
The firft invention of the camera obfeura is aferibed to 
John Baptifla Porta. See his Magia Naturalis, lib. xvii. 
cap. 6, where he largely deferibes the effedts of it. This 
•apparatus ferves for many ufeful and entertaining purpo- 
fes. For example, it is very ufeful in explaining the na¬ 
ture of vifion: it exhibits very diverting fights or fpedta- 
cles; fhewing images perfectly like their objedts, clothed 
in their natural colours, but more intenfe and vivid, and 
at the fame time accompanied with all her motions; the 
leaves quiver, the boughs move, the birds fly, &c. an ad¬ 
vantage which no art can imitate : and, by this inflrttmerit, 
a perfon unacquainted with painting, or drawing, may de¬ 
lineate objedls with the greateft accuracy of drawing and 
colouring. The theory and principle of this inflrument 
may be thus explained. If any objedt A B radiate thro’ 
a fmall aperture L, upon a white ground oppofite to it, 
within a darkened room, or box, &c. the image of the ob¬ 
jedt will be painted on that ground in an inverted fituation. 
For, by the fmallnefs of the aperture, the rays from the 
objedt will crofs each other there, the image of the point 
A being at a, and that of B at b ; fo that the whole ob¬ 
jedt A B will appear inverted, as at a b. And, as the cor- 
refponding rays make equal angles on both tides of the 
aperture, if the ground be parallel to the objedt, their 
heights will be to each other diredtly as their diftances 
from the aperture. 
To conjirubl a Camera Obfeura , by which the images of 
external objedts (hall be repreiented diftindtly, and in their 
genuine colours, firft, darken a chamber that has one of 
its windows looking towards a place containing various ob¬ 
jects to be viewed ; leaving only a frnail aperture open in 
one (butter. 2d, In this aperture fit a proper lens, either 
plano-convex, or convex on both fides; the convexity 
forming a fmall portion of a large fphere. But, if the 
aperture be made very fmall, as of the-ftze of a pea. the 
Vol. III. No. l S3 . 
C A M 653 
objedts will he reprefented oven without any lens at all. 
3d, At a proper diftance, to be determined by trial , ftretch 
a paper orwhite cloth, unlefs there be a white wall at that 
diftance, to receive the images'of r he objedts : or the bell 
way is to have fome philter of Paris caft on a convex 
mould, fo as'to form a concave fmooth furface, and of a 
curvature and (ize adapted to the lens, to be placed occa- 
(ionally at the proper diftane'e. 4th, If it be rather de/tred to 
have the objects appear erect, inftead of inverted, this may 
be done either by placing a concave lens between the cen¬ 
tre and the focus of the firft lens; or, by reflecting the 
image from a plane fpeculum inclined to the horizon in an 
angle of 45 0 ; or by having two lenfes included in a draw- 
tube, inftead of one in the fcioptic ball. That the images 
be clear and diftindt, it is neceftary that the objedts be il¬ 
luminated by the fun’s light fnfhihg upon them from the 
oppofite quarter : fo that, in a weftern profpedt, the images 
will be belt feen in a forenoon, an eaftern profpedt in the 
afternoon, and a northern profpedt about noon ; a fouthern 
afpedt is the leaft eligible of any. But the.beft way is to 
fix the lens in a proper frame on the top of a building ; 
and it nutft be made to move ealilv round in all directions, 
by a handle extended to the perfon'who manages the in- 
ftrument ; for then, the images being thrown down into a 
dark room immediately below it, upon a horizontal round 
plafter-of-Paris ground, a view of every objedt quite around 
will be eafily taken in the fpace of a few minutes; as is the 
cafe of the excellent camera obfeura placed on the tdp of 
the Royal Obfervatory at Greenwich. The objedts will 
be feen brighter, if the fpedtator firft wait a few minutes 
in the dark. Care (liould alfo betaken, that no light 
efcape through any chinks; and, that the ground be not 
too much illuminated. It may further be obferved, that, 
the greater diftance there is between the aperture and the 
ground, the larger the images will be; but theyr at the fame 
time the brightnefs is weakened more and more with the 
increafe of diftance. 
To conflrubl a Portable Camera Obfeura. Provide a fmall 
box or cheft of dry wood, about ten inches broad, and 
two feet long or more, according to the ftze of the lenfes. 
In one fide of. it, as B D, fit a Aiding tube E F with two 
lenfes; or, to have the image at a lefs diftance from the 
tube, with three lenfes, convex on botli fides; the diame¬ 
ter of the two outer ones to be about feven inches, but 
that of the inner to be lefs, as four and three-quarters or 
five inches. At a proper diftance, within the box, let up 
perpendicularly an oiled paper G H, fo that images thrown 
upon it may he feen through. In the oppofite fide, at I, 
make a round hole, for a perfon to look conveniently thro’ 
with both eyes. Then, if the tube F be.turned towards 
the objects, and the lenfes be placed by trials at the pro¬ 
per diftance, by Aiding the tube in and out, the objects 
will be feen delineated on the paper, eredt as before. The 
machine may be better accommodated for drawing, by 
placing a mirror to pafs from C to G ; for this will refledi 
the image upon a rough glafs plane, or an oiled paper, 
placed horizontally at A B ; and a copy of it may then be 
traced off with a black-lead pencil. 
Another Portable Camera Obfeura may be thus made. On. 
the top of a box or cheft raife a little turret H I, open 
towards the objedl AB, as in the annexed figure. Behind 
the aperture, incline a fmall mirror a b at an angle of 45 0 , 
8 D ^ to 
1 
