6/0 
O P T 
between your eye and the point q: then lay the arch flat 
upon the paper on the board, as at the dotted part ST, 
and the interfedtion of the threads will be at W. Mark 
the point W on the paper with the dot of a black-lead 
pencil, and fet the arch upright again as before. Then 
look through the hole z, and move the Aiders N and O 
till the interfedtion of the threads comes between your^ 
eye and any other point of the houfe, as p: then lay the 
arch again to the paper, and make a pencil-mark thereon 
at the interfedlion of the threads, and draw a line from 
that mark to the former one at W ; which line will be a 
true perfpedtive reprefentation of the corner pq of the 
houfe. 
Proceed in the fame manner, by bringing the interfec- 
tion of the threads fucceflively between your eye and 
other points of the outlines of "the houfe, as r, s, &c. and 
put down the arch to mark the like points on the paper, 
at the interfeflion of the threads: then connedl thefe 
points by ftraight lines, wdiich will be the perfpedtive 
outlines of the houfe. In like manner find points for the 
corners of the door and windows, top of the houfe, chim¬ 
neys, &c. and draw the finifhing lines from point to point: 
then (hade the whole, making the lights and (hades as you 
fee them on the houfe itfelf, and you will have a true per- 
Ipective figure of it. Great care mud be taken, during 
the whole time, that the pofition of the machine be not 
((lifted on the table; and, to prevent fuch an inconve¬ 
nience, the table (hould be very ftrong and Heady, and the 
machine fixed to it either by fcrews or clamps. 
In the fame way, a landfcape, or any number of objedts 
within the field of view through the arch, may be deli¬ 
neated, by finding a fufflcient number of perfpedtive 
points on the paper, and connedting them by ftraight or 
curved lines as they appear to the eye. And, as this 
makes every thing in perfpedtive equally eafy, without 
taking the trouble to learn any of the rules for drawing, 
the operations muft be very pleafing and agreeable. Yet, 
as fcience is (till more fo, we would by all means recom¬ 
mend it to our readers to learn the rules for drawing par¬ 
ticular objedls ; and to draw landlcapes by the eye, for 
which, we believe, no perfpedtive rules can be given. 
And, although any thing may be very truly drawn in 
perfpedtive by means of this machine, it cannot be faid 
that there is the lead degree of fcience in going that way 
to work. 
The arch ought to-be at lead a foot wide at bottom, 
that the eye at Z may have a large field of view through 
it: and the eye (hould then be, at leaft, 10J inches from 
the interfedtion of the threads at P when the arch is fet 
upright; for, if it be nearer, the boundaries of view at 
the fides near the foot of the arch will fubtend an angle 
at Z of more than 60 degrees, which will not only (train 
the eye, but will alfo caufe the outermoft parts'of the 
drawing to have a difagreeable appearance. To avoid 
this, it will be proper to draw back the (tiding bar I, till 
Z be 144 inches diftant from P; and then the whole field 
of view, through the foot-wide arch, will not fubtend an 
angle to the eye at Z of more than 43 degrees ; which will 
give a more eafy and pleafant view, not only of all the 
objedts themfelves, but alfo of their reprefentations on 
the paper whereon they are delineated. So that, what¬ 
ever the width of the arch be, the diftance of the eye 
from it fliould be in this proportion: As 12 is to the 
width of the arch, fo is 14J to the diftance of the eye 
(at Z) from it. 
a. Mr. Peacock likewife invented three Ample inftru- 
ments for drawing architecture and machinery in per¬ 
fpedtive, of which the reader will find (ketches and de- 
(criptions in the 75th volume of the Philofophical 
•Tranfadtions. Thefe defcriptions are not inferted here, 
becaufe we do not think the inftruments fuperior to that 
defcribed by Fergufon, and becaufe we with that our rea¬ 
ders who have occafion to draw, may make themfelves fo 
mugh mafters of the art of perfpedtive as to be above the 
aid of fuch mechanical contrivances. But, for the fake of 
1 
I c s. 
thofe whofe opportunities of improvement in the art do 
not enable them to practife it without fuch helps, we annex 
the following defcription of an inftrument invented for 
this purpofe by Dr. Wollafton, to which he has given the 
name of camera lucida. 
“ Having a ftiort time fince (fays the author), anvufed 
myfelf with attempts to (ketch various interefting views, 
without an adequate knowledge of the art of drawing, 
my mind was naturally employed in facilitating the 
means of transferring to paper the apparent relative pofi- 
tions of the objects before me; and I am in hopes that 
the inftrument which I contrived for this purpofe may be 
acceptable even to thofe who have attained to greater pro¬ 
ficiency in the art, on account of the many advantages it 
pofl'effes over the camera obfcura. The principles on 
which it is conftrudted will probably be molt diftindtly 
explained by tracing the fuccefiive fteps by which I pro¬ 
ceeded in its formation. While I look diredtly down at 
a (lieet of paper on my table, if 1 hold between my eye 
and the paper a piece of plain glafs, inclined from me 
downwards at an angle of 45 0 ,1 fee by refledlion the view 
that is before me, in the fame diredtion that I fee my 
paper through the glafs. I might then take a (ketch of 
it; but the pofition of the objedts would be reverfed. 
To obtain a direft view, it is neceffary to have two reflec- , 
tions. The tranfparent glafs muft for this purpofe be in¬ 
clined to the perpendicular line of fight only the half of 
45 0 , that it may refiedt the view a fecond time from a 
piece of looking-glafs placed beneath it, and inclined 
upwards at an equal angle. The objects now appear as 
if feen through the paper in the fame place as before ; but 
they are diredt inftead of being inverted; and they may 
be difcerned in this manner lufficiently well for deter¬ 
mining the principal petitions. The pencil, however, 
and any objedt which it is to trace, cannot both be feen 
diftindtly in the fame ftate of the eye, on account of the 
difference of their diftances ; and the efforts of fuccefiive 
adaption of the eye to one or to the other, would become 
painful if frequently repeated. In order to remedy this 
inconvenience, the paper and pencil may be viewed 
through a convex lens of fuch a focus, as to require n'o 
more effort than is neceffary for feeing the diftant objects 
diftindtly. Thefe will then appear to correfpond with the 
paper in diftance as well as direction; and may be drawn 
with facility, and with any defired degree of preciiion. 
“ This arrangement of glaffes will be beft underftood 
from infpedting fig. 3. w'herein ah is the tranfparent glafs ; 
he the lower refledtor; bd a convex lens (of 12 inches fo¬ 
cus) ; c the pofition of the eye; and fghe the courfe of 
the rays. But in Come cafes a different conftrudtion will 
be preferable. Thofe eyes, which without afilftance are 
adapted to feeing near objedts alone, will not admit the 
ule of a convex glafs ; but will on the contrary require 
one that is concave to be placed in front, to render the 
diftant objedts diftindt. The frame for a glafs of this 
conftrudtion is reprefented at ih, fig. 4. turning upon the 
fame hinge at h with a convex glals in the frame Im, and 
moving in.fuch a manner, that either of the glades may 
be turned alone into its place, as may be neceffary to (hit 
any eye that is long or (hort lighted. Thofe perfons, 
however, whofe fight is nearly perfedt, may at pleafure 
ufe either of the glaffes. 
“ The inftrument reprefented in that figure differs 
moreover in other refpedfs from the foregoing, which I 
have chofen to deferibe firft, becaufe theadlion of the re- 
fiedtors there employed would be more generally under¬ 
ftood. But thofe who are converfant with the fcience of 
optics will perceive the advantage that may be derived in 
this inftance from prifmatic refledtion; for, when a ray 
of light has entered a folid piece of glafs, and falls from 
within upon any furface, at an inclination of only twenty- 
two or twenty-three degrees, as above fuppofed, the re- 
fradtive power of the glafs is fuch as to fuffer none of that 
light to pafs out, and the furface becomes in this cafe the 
ruoft brilliant refledtor that can be employed. 
“Fig. 
