#> CATO? 
tificial contrivance of catopfrical machines, exhibited by 
fhowmen and others. The general principles by vvhich 
they aft are fufficiently explained above ; a bare defcrip- 
tioii of their conftruCtion and ufes is all that is further re- 
quiihe to inio'rm the reader of the metliod of exhibiting 
thefe experiments, and of the confiderable entertainment 
they are capable of affording to a feleft company. And, 
although thefe tilings may not to the philofopher be 
deemed intereffingor tifeful, yet it maybe remarked, that 
from fuch inferior amufements have arifen many difeove- 
ries of the greateft importance to mankind. 
CATOPTRIC ILLUSIONS. 
Take a glafs decanter or bottle, as at A, fig. 5, in the 
annexed engraving, and fill it with water to the point B; 
leave the upper part BC empty, and cork it in the ufu'al 
manner. Place this, bottle oppofite a concave mirror, and 
beyond its focus, that it may appear revet fed, and before 
the mirror, place yourfelf /fill farther diftant from the 
bottle, and it will appear to you in the inverted fituation, 
a, b,c, fig-5- Now it is remarkable in this apparent bottle, 
that the water, which, according to all the laws of catop¬ 
trics, and all the experiments made on other objects, fiiould 
appear at A B, appears on the contrary at b c, and confe- 
quently the part A B appears empty. If the bottle be in¬ 
verted and placed before the mirror, as in figure 6, its 
image will appear in its natural ereft pofition, and the 
•water, which is in reality at BC, will appear at a b. If, 
while the bottle is inverted, it be uncorked, and the wa¬ 
ter buffered to run gently out, it will appear, that while the 
part BC is emptying, that of a b in the image is filling: and 
what is likewife very remarkable,- as foon as the bottle is 
empty the illufion ceafes, the image alfo appearing entirely 
empty. If the bottle likewife be quite full, there is no il¬ 
lufion. If, While the bottle is held inverted, and partly 
■empty, feme drops of water fall from the bottom A towards 
BC, it feerns in the image as if there were formed at the 
bottom of the part a b bubbles of air that rofe from a to b, 
which is the part that feerns full of water. The remarka¬ 
ble circumffances in this experiment are, firff, not only to 
fee an object where it is not, but alfo where its image is 
not; and fecondly, that of two objects which are really in 
the fame place, as the furface of the bottle and the water 
it contains ; the one is feen at one place, and the other at 
another; and to fee the bottle in the place of its image, 
and the water where neither it nor its image are. It is 
fuppo.fed that this illufion arifes partly from our not being 
accuftomed to fee water fufpen-ded in a decanter with the 
neck downwards, and partly from the refemblance there is 
between the colour of air and that of water. 
The CATOPTRtC Cistula. 
On the hexagonal or fix-fided plane ABCDEF, fig. 7, 
draw fix femudiameters, GA, GB, GC, GD, GE, GF ; 
and on each of thefe, place perpendicularly two plain mir¬ 
rors, which muff join exactly at the centre G, and which, 
placed back to back, muff be as thin as poffible. Decorate 
the exterior boundary of this piece (which is at the extre¬ 
mity of the angles of the hexagon) with fix columns, that 
at tlie fanie time ferve to fupport the mirrors, by grooves 
formed on their inner fides. (See the profile H.) Add to 
thefe columns their entablatures, and cover the edifice as 
you think proper. In each of thefe fix triangular fpaces, 
contained between two mirrors, place little figures of paffe- 
board, in relief, reprefenting fuch objects, as, when feen in 
an hexagonal form, will produce an agreeable effeift. To 
thefe add fmall figures of enamel, and take particular care 
to conceal, by fome objecl that has relation to the fubjeift, 
the place where (he mirrors join, which, as we have faid 
before, all meet in the common centre G. When von 
look into any one of the fix openings of this palace, the ob¬ 
jects there contained, being repeated fix times, will feem 
entirely to fill up rhe whole of the building'. This ill 11 - 
(ion will appear very remarkable; efpecially if the objefts 
made choice of are properly adapted to the effect that is to 
TRIG S. 
be produced by the mirrors. If you placebetwceirtwo of 
thefe mirrors part of a fortification, as a curtain and two 
demi-baftions, you will fee an entire citadel, with its fix 
baftions. Or if you place part of a ball-room, ornamented 
with chandeliers and figures in -enamel, all thofe objects 
being here multiplied, will afford a very biilliant effect. 
The Catoptrical Pa radox. 
This illufion is produced by a double perfpeftive, as 
fhewn at fig. 8, in the plate. <£)ne of the peripedtives of 
the inffrument being placed before the eye, an ofcjedt will 
be feen directly through both ; a board, A,or other opaque 
body, being interpofed, will not make the leaf! ob (fruition 
to the rays'; and the obferver will be furprifed, as it were, 
that he fees through a perfpective having the property of 
penetrating either folid metal or wood. The conftruition 
of this inffrument is chiefly from four fmall plane reflect¬ 
ing glafs mirrors, a, b, c, d, fig. 9, of which a and d are 
placed at an angle of 45 degrees in the two perfpectives, 
and cand d parallel thereto in the trunk below, which is fo 
formed as to appear like a folid handle to the perfpeitives,- 
It is evident, from the principle of catoptrics, that the 
objeit T, falling on the firff mirror d, will be refieftei 
down to c, on b, and up to a, and thence out to the eye, 
-giving the appearance of the ffaaight lineal direction, a i. 
The Endless Gallery. 
This is conftrinfted in a mahogany or other box, about 
eighteen inches in length, twelve inches broad, and nine 
inches deep, as fhewn at fig. 10 ; but thefe dimenlions are 
not effential, and may be altered at pleafure, except in 
about thefe proportions. In tlie infide of this box, and 
againft each of its two oppofite faces, A and B, place a 
plane true-ground glafs mirror, as free from veins as pof¬ 
fible, of the dimenfions nearly equal to the faces, only al¬ 
lowing a fmall fpace for a tranfparent paper, or other cover 
at top. From the middle of the mirror C, placed at B, 
take off neatly a round furfice of the filvering, about one 
inch and an half in diameter, where in the fide of the box 
muff be cut a correfpondent hole, not more in diameter 
than that of the glafs. The top fhould be of glafs covered 
with gauze, or of oiled tranfparent paper, it being effential 
that as much light as poffible be admitted into the box. 
Within this box on the two long fides muff be cut or 
placed two grooves, at E and P, to receive various draw¬ 
ings or paintings, as hereafter deferibed. It is bell to cut 
many grooves in the fides for the reception of fuch a vari¬ 
ety of objects as may be defired. Two drawings of any 
perfpeftive fubjeft muff be made on the two oppofite faces 
of a pafteboard, fuch as reprefented at fig. n and 12; as 
foreffs, gardens, colonnades, &c. After having cut the 
blank parts carefully out, place them in the two grooves, 
E and F, of the box. Paint alfo on two other boards of 
the fame dimenfions fimilar fu-bjecls, but only on one fide, 
obferving that the one which is to be placed on the glafs, 
containing the opening C, fiiould have nothing drawn there 
to prevent the fight, and that the other for the oppofite 
fide, D, fhould alfo not be very full of figures. After it is 
nearly cut out and placed on the glafs, it fhould cover but 
a fmall part of it. The other board fhould be cut out, 
and alfo contain but few figures, fo as only to be neceflhry" 
to difguife the repetition of the aperture C, without which 
it would appear on the glafs D; this is to be applied-to the 
glafs D. The top is then to be covered up with its tranf- 
parent cover, and the inffrument is ready.for ufe. The effedt 
is very entertaining and ftriking. The eye, being applied 
to the opening C, will perceive the various objects drawn 
on the f'cenes reflected in a very fucceflive and endlefs 
manner, by being reflected from one of the mirrors upon 
that which is oppofite; as for inffance, if they were trees, 
it would appear an entire grove, very long, and of which 
there would appear no end; each of the mirrors repeating 
the objeftsmore faintly, as the reflections are more nume¬ 
rous, and thereby contributing (till more to the illufion. 
Ingenuity will fugged a variety of entertaining figures of 
1 men. 
