OPT 
Of the Magic Lantern. 
ABCD, fig. 2. reprefents a tin box, or lantern, in the 
fore part of which is a Hiding tube, furnilhed with a 
double convex lens EF. Between the lantern and the 
lens, a fmall Space, qp, is left, to admit a thin plate of 
glafs, upon which inverted figures are painted in trans¬ 
parent colours. 
When this inftrument is ufed, the lamp K being 
lighted, and the room darkened, the tube is moved till 
qp is farther from the lens than its principal focus f; and 
consequently PQ, an inverted image of pq, or an ereCt 
image of the figure intended to be represented, is formed 
at Some diftance from the lens, and painted in its proper 
colours upon a Screen placed at the concourSe of the re- 
fraCted rays. 
Sometimes a refleCtor is placed behind the lamp, or a 
convex lens before it, for the purpofe of throwing a greater 
quantity of light upon pq. 
Cor. If the Screen and lantern-be fixed, and their dis¬ 
tance exceed four times the focal length of the lens, the 
image may be thrown upon the Screen, by moving the 
lens nearer to, or farther from, pq, as the cafe requires. 
For, qf : qE :: </E : qQ, or qE--/E : qE :: qE : qQ ; 
in which proportion there is only one unknown quan¬ 
tity, qE, which may be determined by the Solution of a 
quadratic equation whofe roots are poffible, except the 
diftance qQ be leSs than four times the focal length of the 
lens EF. 
Exhibitions of the appearance of SpeCtres have Some¬ 
times been formed on the principles of the magic lan¬ 
tern, with certain variations. The moft ftriking decep¬ 
tion of this kind is the P/iantafmagoria, which, Some 
winters ago, formed one of the principal public amufe- 
ments at Paris and London. This exhibition was con¬ 
trived by Mr. Philipfthal, and was conducted in a fmall 
theatre, (the Lyceum,) all the lights of which were 
removed, except one hanging lamp; and this could be 
drawn up, So that its flame was perfectly enveloped in a 
cylindrical chimney, or opaque fliade. In this gloomy 
and wavering light the curtain was drawn up, and pre¬ 
sented to the Spectators a Sort of cave, with fkeletons and 
6ther figures of terror painted or moulded in relievo on 
the Sides or walls. After a fhort interval, the lamp was 
drawn up into its chimney, and the Spectators were in 
total darknefs, interrupted only by flafhes of lightning 
Succeeded by peals of thunder. Thefe phenomena were 
followed by the appearance of figures of departed men, 
ghofts, fkeletons, tranfmutations, &c. &c. Several figures 
of celebrated men were thus exhibited with various trans¬ 
formations ; Such as the head of Dr. Franklin Suddenly 
converted into a Skull, &c. Thefe were Succeeded by 
phantoms and various terrific figures, which were Some¬ 
times Seen to contract gradually in all their dimenfions, 
till they became extremely fmall, and then vaniflied; 
while at others, inftead of Seeming to recede and then 
vanifh, they were, to the furprife and aftonifhment of the 
Spectators, made Suddenly to advance, and then disap¬ 
pear by Seeming to fink into the ground. 
The principal part of thefe phenomena was produced 
by a modification of the magic lantern, having all its 
parts on a large Scale, and placed on that fide of a femi- 
tranfparent Screen' of taffeta which was oppofite- to the 
Spectators, inftead of the fame fide, as in the ordinary 
exhibitions of the magic lantern. To favour the decep¬ 
tion, the Aiders were made perfectly opaque except in 
thole places that contained the figures to be exhibited, 
and in thefe light parts the glafs was covered with a more 
or lefs transparent tint, according to the effeCt required. 
The figures for thefe purpofes have alfo been drawn with 
water- colours on thin paper, and afterwards varnifhed. To 
imitate the natural motions of the objeCts represented, 
Several pieces of glafs, placed behind each other, were oc¬ 
casionally employed. By removing the lantern to different 
diftances, and at the fame time altering more or lefs the 
Vol.XVII. No. 1205. 
I C S. 653 
pofition of the lens, the images were made to increafe or 
diminifh, and to become more or lefs diftinft at the plea¬ 
sure of the exhibitor; So that, to a perfon unaccuftomed 
to the effect of optical inftruments, the figures appeared 
actually to advance and retire. In reality, however, fi¬ 
gures exhibited in this way become much brighter as 
they are rendered Smaller; while, in nature, the imperfeCf 
transparency of the air caufes objeCts to appear fainter 
when they are remote than when they are nearer the ob¬ 
server. Sometimes, by throwing a ftrong light on an 
objeCt really opaque, or on a living perfon, its image was 
formed on the curtain, retaining its natural motions; 
but in this cafe the objeCt muft have been at a consider¬ 
able diftance, otherwise the images of its nearer and 
remoter parts could never be Sufficiently diftinct at once, 
as the refraCtion muft either be too great for the re¬ 
moter, or too Small for the nearer, parts; and there muft 
alfo be a Second lens placed at a Sufficient diftance from 
the firft, to allow the formation of an inverted image 
between them, and to throw a Second picture of this 
image on the Screen in its natural ereCt pofition, unlefs 
the objeCt be of Such a nature that it can be inverted 
without inconvenience. 
Mr. Ezekiel Walker has lately conftruCted a new opti¬ 
cal inftrument, calculated for affording entertainment to 
thofe wdro derive pleafure from optical illufions. This 
inftrument is called phantafmafcope, and is So contrived, 
that a perfon Handing before it fees a door opened, and 
a phantom make its appearance, coming towards him, 
and increafing in magnitude as it approaches, like thofe 
in the phantafmagoria. When it has advanced about 
three feet, it appears of the greateft magnitude, and, as it 
retires, becomes gradually contracted in its dimenfions, 
till it re-enters the machine, when it totally vanifhes. 
This phantom appears in the air like a beautiful paint¬ 
ing; and has Such a rich brilliancy of colouring as to ren¬ 
der it unneceftary to darken the room. On the contrary, 
this aerial picture is Seen with rather greater perfection 
when the room is illuminated. 
Mr. Walker has fliown how this inftrument may alfo 
be employed to exhibit Several phenomena in the hea¬ 
vens; as, for example, the appearance of Jupiter and his 
Satellites, and the colour of Mars and the Moon. See 
Nicholfon’s Journal, vol. i. 8vo. Young’s LeCt. on Nat. 
vol. i. Phil. Mag. vol. xxvii. 
Of the Aftronomical Telefcope. 
The aftronomical telefcope confifts of two convex 
lenfes, whofe axes are in the fame right line, and whofe 
diftance is equal to the Sum of their focal lengths. The 
common axis of the two lenfes is called the axis of the 
telefcope. The lens which is turned towards the objeCt 
to be viewed, and which has the greater focal length, is 
called the objett-glafs. The lens to which the eye is ap¬ 
plied, is called the eye-glafs. 
Prop. I. A diftant object may be Seen diftinCtly through 
the aftronomical telefcope ; and the angle which it Sub¬ 
tends at the centre of the eye, when thus Seen, is to 
the angle which it Subtends at the naked eye, as the 
focal length of the objeCt-glafs to the focal length of 
the eye-glafs. 
Let L and E, fig. 3, be the centre of the two glafles; 
QP an objeCt towards which the axis of the telefcope is 
directed ; and to diftant, that the rays which flow from 
any one point in it, and fall upon the objeCt-glafs L, may 
be confidered as parallel. Then qp, an inverted image of 
QP, will be formed in the principal focus of the glafs L, 
and contained between the lines QL</, and EEp ; and, 
becaufe LE is equal to the Sum of the focal lengths of the 
two glafles, pq is in the principal focus of the glafs AB; 
confequentlyj/oymay be Seen diftinCtly through'this glafs, 
if the eyerof the obferver be able to colleCt parallel rays 
upon the retina. Produce PLp till it meets the eye-glafs 
8 D , in 
