669 
OPT 
of the fimpleft figures of this writer,.as by this means the 
-myftery of this art will be fufficiently unfolded. 
Upon the cylinder of paper,or pafteboard, ABCD, fig. 25, 
draw whatever is intended to be exhibited, as the letters 
IHS. Then with a needle make perforations along the whole 
outline; and, placing a candle, G, behind this cylinder, 
mark upon the ground-plane the ffiadow of them, which 
will be diftorted more or lefs, according to the pofition of 
the candle or the plane, &c. This being done, let the 
pitture be an exaft copy of this diftorted image, let a. me¬ 
tallic fpeculum be fubftituted in the place of the cylinder, 
and let the eye of the lpeftator have the fame pofition be¬ 
fore the cylinder that the candle had behind it. Then, 
looking upon the fpeculum, he will fee the diftorted 
image reftored to its proper fhape. The re-formation of the 
image, he fays, will not eafily be made exaft in this me¬ 
thod, but it will be fufficiently fo to anfwer the purpofe. 
Other methods, more exadt and geometrical than this, 
were found out afterward; fo that thefe piftures could 
be drawn by certain rules, without the ufe of a candle. 
Schott quotes one of thefe methods from Bettinus, another 
from Herigonius, and another from Kirc’ner, which may 
be feen in his Magia, vol. i. He alfo gives an account of 
the methods of re-forming pictures by fpeculums of coni¬ 
cal and other figures. But, inftead of copying any of 
thefe methods from Schott or Bettinus, we ffiail prefent 
our readers with that which Dr. Smith has given us in his 
Optics, vol. i. as, no doubt, the beft, and from which any 
perfon may eafily make a drawing of this kind. The fame 
defcription anfwers to two mirrors, one of which, fig. 26, 
is convex, and the other, fig. 27, is concave. 
In order to paint upon a plane a deformed copy 
ABCDEKIHGF, fig. 26, of an original pifture, which 
ffiail appear regular, when feen from a given point O, 
elevated above the plane, by rays reflefted from a poliffied 
cylinder, placed upon the circle hip, equal to its given 
bafe ; from the point R, which muft be fuppofed to lie 
perpendicularly under O, the place of the eye, draw two 
lines Rtf, Re, which ffiail either touch the bafe of the cy¬ 
linder, or elfe cut off two fmall equal fegments from the 
iides of it, according as the copy is intended to be more 
or lefs deformed. Then, taking the eye, raifed above R, 
to the given height RO, fomewhat greater than that of 
the cylinder, for a luminous point defcribe the fliadow 
avhf (of a fquare, or parallelogram (landing upright upon 
ae as a bafe, and containing the picture required) any¬ 
where behind the arch hip. Let the lines drawn from R 
to the extremities and divifions of the bafe a, b, c, d, e, 
cut the remoteft part of the thadow in the points f, g, h, 
i, h, and the arch of the bafe in /, m, n, o, p; from which 
points draw the lines IAF, wBG, «CH, oDI, pEK, as if 
they were rays of light that came from the focus R, and 
were reflected from the bafe rtrip ■, fo that each couple, IA, 
/R, produced, may cut off equal fegments from the circle. 
Laftly, transfer the lines laf, mbg , &c. and all their parts 
in the fame order, upon the refpeftive lines ZAF, wtBG, 
See. and having drawn regular curves, by eftimation, 
through tile points A, B, C, D, E, through F, G, H, I, X, 
and through every intermediate order of points ; the 
figure ACEKHF, fo divided, will be the deformed copy 
of the fquare, drawn and divided upon the original pic¬ 
ture, and will appear fimilar to it, when feen in the 
poliffied cylinder, placed upon the bafe hip, by the eye in 
its given place O. 
The practical methods of drawing thefe images feem 
to have been carried to the greateft perfeftion by J. 
Leopold, who, in the Afta Lipfienfia for the year 1712, 
has deferibed two machines, one for the images to be 
viewed with a cylindrical, and the other with a conical, 
mirror. The perfon poffeffed of this inftrument has no- 
filing to do but to take any print he pleafes; and, while 
lie goes over the outlines of it with one pen, another 
traces the anamorphofis. 
By methods of this kind, groves of trees may be cut, 
fo as to reprefent the appearance of men, liorfes, and other 
Vol. XVII. No 1206. 
ICS. 
objefls from fome one point of view, which are not at all 
difcernible in any other. This might eafily be effected 
by one perfon placing himfelfin any particular fituation. 
and giving directions to other perfons what trees to lop, 
and in what manner. In the fame method it has been 
contrived, that buildings of circular and other forms, 
and alfo whole groups of buildings confiding of walls at 
different diftances and with different pofitions'to one ano¬ 
ther, ftiould be painted fo as to exhibit the.exaCt repre- 
fentation of particular objeCls, which could only be per¬ 
ceived in one fituation. Bettinus has illuftrated this 
method by drawings in his “ Aparia.” 
PERSPECTIVE INSTRUMENTS-. Plate XX. 
The inftruments neceffary for drawing according to 
the rules of perfpeftive, are—a tee fquare, a parallel ruler, 
a drawing-board (which is only a fmooth board made ex- 
aftly fquare), a leCtor, a protraftor; to which may be 
added a drawing-pen, and a black-lead pencil. 
1. The perj'peStive machine is an inftrument by which any 
perfon, without the help of the rules of art, may deli¬ 
neate the true perfpeftive figures of objefts. Mr. Fergu- 
fon has deferibed a machine of this fort, of which he 
aferibes the invention to Dr. Bevis. Fig. 1 is a plan of 
this machine, and fig. 2 is a reprefentation of it when 
made ufe of in drawing diftant objefts in perfpeftive. 
In fig. 1, ahef is an oblong fquare board, reprefented by 
ABEF in fig. 2. x and y (X and Y) are two hinges on 
which the part cld (CLD) is movable. This part confifts 
of two arches or portions of cml (CML) and dnl (DNL) 
joined together at the top l (L), and at bottom to the 
crofs bar dc (DC), to which one part of each hinge is 
fixed, and the other part to a flat board, half the length 
of the board ahef (ABEF), and glued to its uppermoft 
fide. The centre of the arch dnl is at c. On the outer 
fide of the arch dnl is a Aiding piece n (much like the nut 
of the quadrant of altitude belonging to a common 
globe), which may be moved to any part of the arch be¬ 
tween d and l: and there is fuch another Aider o on the 
arch end, which may be fet to any part between c and l. 
A thread cpn (CPM) is ftretched tight from the centre c 
(C) to the Aider n (N), and fuch another thread is 
ftretched from the centre d (D) to the Aider o (O) ; the 
ends of the thread being faftened to thefe centres and 
Aiders. Now it is plain, that, by moving thefe Aiders on 
their refpeftivearches, the interfeftionp (P) of the threads 
may be brought to any point of the open fpace within the 
arches. In the groove h (K.) is a ftraight Aiding bar i 
(I), which may be drawn further out, or puffied further 
in, at pleafure. To the outer end of this bar I (fig. 2.) is 
fixed the upright piece HZ, in which is a groove for re¬ 
ceiving the Aiding piece Q. In this Aider is a fmall hole 
v for the eye to look through, in ufing the machine : and 
there is a long flit in HZ, to let the hole r be feen through 
when the eye is placed behind it, at any height of the hole 
above the level of the bar I. 
Now, fuppofe you wanted to delineate a perfpeftive 
reprefentation of any diftant objeft, as the houfe qsrp , 
(which we muft imagine to be a great way off, without 
the limits of the plate.) Place the machine on a rteady 
table, with the end EF of the horizontal board ABEF 
toward the houfe, fo that, when the arch DLC is fet up¬ 
right, the middle part of the open fpace (about P) within 
it may be even with the houfe when you place your eye 
at Z, and look at the houfe through the fmall hole x. 
Then fix the corners of a fquare piece of paper with four 
wafers on the furface of that half of the horizontal 
board which is neareft the houfe ; and all is ready for 
drawing. 
Set the arch upright, as in the figure; which it will be 
when it comes to the perpendicular fide t of the upright 
piece xy fixed to the horizontal board behind D. Then 
place your eye at Z, and look through the hole z at any 
point of the houfe, as q, and move the Aiders N and O 
till you bring the interleftion of the threads at P direftly 
8 H between 
