OPTICS. 
a fymmetrical pi£Vure cannot be created by the union of 
two parts apparently diffimilai*. If the eye of the obferver 
is placed near his hand, fo that he can fee diltindTly both 
the hand and its image, the angular magnitude of his 
band is much greater than that of its image; and there¬ 
fore, when the two are united, they cannot form a fym- 
metrical objedt. 
This will be better underftood from fig. 10. When the 
eye is placed at C, the objedt MNOP is cbvioufly nearer 
than its image mnop, and mult therefore appear larger; 
and this difference in their apparent magnitudes will 
increafe as the eye rifes Above the plane of the mirror AE. 
As the eye approaches to E, the diltances of the objedt 
and its image approach to an equality; and, when the eye 
is at E, the objedt MNOP, and its image mnop, are fitu- 
ated at exadtly the fame diftance from the eye, and there¬ 
fore have the fame angular magnitude. Hence it follows 
that, when they are united, they will form a perfectly-fym- 
rnetrical form. 
When the eye is placed in the plane of both the mir¬ 
rors, the field of view arifing from the multiplication of 
the feet or AOB, fig. i. will be perfectly circular; but, as 
the eye rifes above the plane of both the mirrors, this 
circle will become a fort of ellipfe, becoming more and 
more eccentric as the eye comes in front of the mirrors, 
or rifes in the direction EC. If the obferver were infi¬ 
nitely diftant, tbefe figures would be corredt ellipfes; 
but as the eye, particularly when the mirrors are broad, 
mult be nearly twice as far from the lall-refledted fedtor 
as from the'fedtor feen by diredt vifion, the field of view, 
and confequently every pattern which it contains, mult 
be diftorted, and deftitute of beauty, from this caufe alone. 
Hitherto we have alluded only to fymmetry of form ; 
but it is manifeft that, before the union of two fimilar 
forms can give pleafure to the eye, there mull be alfo a 
fyrnmetry of light. If the objedt MNOP, fig. io. is white, 
and its image mnop black, they cannot poffibly form, by 
their combination, an agreeable pidture. As any confi- 
derable difference in the intenfity of the light will deftroy 
the beauty of the patterns, it becomes a matter of indif- 
penfable importance to determine the pofition of the eye 
which will give the greateft poffible uniformity to the 
different images of which the pidlure is compofed. 
It has been afcertained by the accurate experiments of 
Bouguer, that, when light is refledted perpendicularly 
from good plate-glafs, only 25 rays are refledted out of 
1000 ; that is, the intenfity of the light of any objedt feen 
perpendicularly in plate-glafs, is to the intenfity of the 
light of its image as 1000 is to 25, or as 40 to 1. When 
the angle of incidence is 6o°, the number of refledted rays 
is 112, and the intenfities are nearly as 9 to 1. When the 
angle of incidence is 87^, the number of refledted rays is 
584, and the intenfities are nearly as 17 to 10, fo that ti e 
luminoufnefs of the objedt and its image approach rapidly 
to an equality. It is therefore clear, that, in order to 
have the greateft uniformity of light in the different 
images which compofe the figure, the eye muff be placed 
as nearly as poffible in the plane of both the mirrors ; 
that is, as nearly as poffible to the angular point. But, 
as it is impradiicable to have the eye exadfly in the plane 
of both mirrors, the images formed by one refledtion muft 
always be lefs bright than the diredl objedt, even at the 
part nearly in contact with the objedt. The fecond and 
third refledtions, See. where the rays fall with lefs obli¬ 
quity, will be ftill darker than the firft ; though this dif¬ 
ference will not be very perceptible when the inclination 
of the mirrors is 30 0 or upwards, and the eye placed in 
the pofition already deferibed. 
It is a curious circumftance, that the pofitions of the 
eye which are neceffary to eftedt a complete union of the 
images—to reprefent fimilar parts of the objedt and its 
images—to obferve the objedt and its image under the 
fame angular magnitude, and to give a maximum intenfity 
of light to the refledted images—fhould all unite in the 
fame point. Had this not been the cafe, the conftrudtion 
of the kaleidofcope would have been impradiicable; and 
hence it will be feen how vain is the attempt to produce 
beautiful and fymmetrical forms from any combination of 
plain mirrors in which this pofition of the eye is not a 
radical and effential principle. 
Thus much of the theory of the kaleidofcope. We 
now proceed to explain its 
Coijh-ufiion and Uje .—In order to conftrudt the kalei¬ 
dofcope in its moft fimple form, we muft procure two 
reflectors, about five, fix, feven^ or eight, inches long. 
Thefe refledtors may be either redtangular plates, or plates 
fhaped like thofe reprefented in fig. 1. having their broad- 
eft ends AO from one to two inches wide. If the re¬ 
fledtors are of glafs, the neweft plate-glafs fhould be ufed, 
as a great deal of light is loft by employing old plate-glafs, 
with fcratches or imperfedtions upon its furface. The 
plate-glafs may be either quick-fiivered or not, or its pof- 
terior furface may be ground, or covered with black wax, 
or varnifli, or any thing elfe that removes its reflective 
power. This, however, is by no means ab olutely necef¬ 
fary ; for, if the eye is properly placed, the refledtions 
from the pofterior furface will fcarcely aftedt the diftindt- 
nefs of the pidlure, unlefs in very intenfe lights. If it 
fhould be thought neceffary to extinguifh as completely 
as poffible all extraneous light that may be thrown into- 
the tube from the pofterior furface of the glafs plates, 
that furface fhould be coated with a varnifli of the fame 
refractive and difperfive power as the glafs. 
If the plates of glafs have been fkilfully cut with the 
diamond, fo as to have their edges perfectly ftraight, and 
free from chips, two of the edges may be placed together, 
as in fig. 1. or one edge of one plate may be placed again ft 
the furface of the other plate, as fliown in fig. 17. But, 
if the edges are rough and uneven, one of them may be 
made quite ftraight, and freed from all imperfections, by 
grinding it upon a flat furface, with very fine emery, or 
with the powder feraped from a hone. When the two 
plates are laid together, fo as to form a perfect jundtion, 
they are then to be placed in a brafs or any other tube, 
fo as to form an angle of 45 0 , 36°, 30 0 , or any even ali¬ 
quot part of a circle. In order to do this with perfedt 
accuracy, diredl the tube containing the refledtors to any 
line, fuch as mn, fig. 2. placed very obliquely to one of 
the refledtors AO, and open or fhut the plates till the 
figure of a ftar is formed, compofed of 8, 10, or 12, fedtors, 
or with 4, 5, or 6, points, correfponding to angles of 45 0 , 
36°, and 30 0 . When all the points of the ftar are equally- 
perfedt, and none of the lines which form the falient and 
re-entering angles difunited, the refledtors muff be fixed 
in that pofition by fmall arches of brafs AB, ab, fig. n. 
filed down till they exadtly fit the fpace between the open 
ends of the plates. The plates muft then be kept in this 
pofition by pieces or wedges of cork or wood, or any 
other fubftance puftied between them and the tube. - The 
greateft care, however, muft be taken that thefe w'edges 
prefs lightly, upon the refledtors ; for a very flight force is 
capable of bending dnd altering the figure even of very 
thick plates of glafs. 
Whefl the refledtors are thus placed in the .tube, as 
in fig. 11. their extremities «E, dE, next the eye, muft 
reach to the very end of the tube, as it is of the greateft 
importance that the eye get as near as poffible to the re¬ 
flectors The other ends of the refledtors AO, BO, muft 
alfo extend .0 the other extremity .of the tube, in order 
that they may be brought into contact with the objedts 
which are to be applied to the inftrument. In ufing 
tranfparent objedts, the cell which contains them may be 
ferewed into the end of the tube, fo as to reach the ends 
of the refledtors, if they happen to terminate within the 
tube; but an inftrument thus conftrudted is incapable of 
being applied to opaque objedts, or to tranfparent objedts 
feen by refledted light. If the plates are narrower at the 
eye-end, as in fig. 11. the angular point E fhould be a 
little on one fide of the axis of the tube, in order that the 
aperture in the centre of the brafs cap next the eye may 
be 
