G40 
OPTICS. 
fore obferved, when the objefits, in my early experiments, 
were fituated at a diftance from the reflectors. From the 
approximation to perfect fymmetry which the figure now 
displayed, compared with the great deviation from fym¬ 
metry which I had formerly obferved, it was obvious that 
tl'.e progreffion from the one eft’efit to the other mult take 
place during the paffage of the objeft from the one pofi- 
ticn to the other; and it became highly probable, that a 
pofition would be found where the lymmetry was mathe¬ 
matically perfedt. By inveftigating this fubjefit optically, 
I discovered th.e leading principles of the kaleidolcope, fo 
far as the inclination of the refleftors, the pofition of the 
object, and the pofition of the eye, were concerned. I 
found, that, in order to produce perfectly beautiful and 
fymmetrical forms, three conditions were necefiary : x. 
That the refleftors fhould be placed at an angle, which 
was an even or an odd aliquot part of a circle, when the 
objeCt was regular, and fimilarly fituated with refpeft to 
both the mirrors; or the even aliquot part of a circle when 
the objeCt was irregular, and had any pofition whatever. 
2. That, out of an infinite number of pofitions for the ob- 
jefit, both within and without the refleftors, there was 
only one where perfeCt lymmetry could be obtained, name¬ 
ly, when the objeCt was placed in contaCt with the ends 
of the reflectors. 3. That, out of an infinite number of 
pofitions for the eye, there was only one where the fym¬ 
metry was perfect, namely, as near as poffible to the an¬ 
gular point, fo that the circular field could be diltinCtly 
feen ; and that this point was the only one out of an infi¬ 
nite number at which the uniformity of the light of the 
circular field was a maximum, and from which the direCt 
and the reflected images had the fame form and the fame 
magnitude, in confequence of being placed at the fame 
dillance from the eye. 
“ Upon thefe principles I conflruCted an inltrument, in 
which I fixed permanently, acrofs the ends of the refleftors, 
pieces of coloured glafs, and other irregular objects; and 
I (hewed the inltrument in this Hate to fome members of 
the Royal Society of Edinburgh, who were much llruck 
with the beauty of its effeCts. In this cafe, however, the 
forms were nearly permanent, only a flight variation was 
produced by varying the pofition of the inltrument with 
refpeCt to the light. 
“ The great ltep, however, towards the completion of 
the inltrument, remained yet to be made; and it was not 
till fome time afterwards, that the idea occurred to me of 
giving motion to objects, Jack as pieces of coloured gjaj's, fc. 
which were either fixed or placed loofe/y in a cell at the end of 
the bjirument. When this idea was carried into execu¬ 
tion, and the refieitors placed in a tube, and fltted-up on 
the preceding principles, the kaleidolcope, in its Jimple 
form, was completed. 
“ In this form, however, the kaleidofcope could not be 
confldered as a general philofopnical inltrument of uni- 
verfal application. The leaft deviation of the objeCt from 
the pofition of fymmetry at the end of the refleCtors, pro¬ 
duced a deviation from beauty and fymmetry in the figure, 
and this deviation increafed with the diftance of the ob¬ 
ject. The ufe of the inltrument was therefore limited to 
objeCts held ciofe to the refleCtors, and conl'equently to 
objeCts whole magnitudes were lefs than its triangular 
aperture. The'next, and by far the molt important, ltep 
of the invention, was to remove this limitation, and to 
extend indefinitely the ufe and application of the inltru- 
ment. This effect was obtained by employing a draw- 
tube, containing a convex lens of fuch a focal length, 
that the images of objeCts, of all magnitudes and at all 
diftances, might be diftinftly formed at the end of the 
refleCtors, and introduced into the pictures created by 
the inltrument in the fame manner as if they had been 
reduced in lize, and placed in the true pofition of fym¬ 
metry. 
“ When the kaleidofcope was brought to this degree of 
perfection, it was impoffible not to perceive that it would 
prove of the higheft fervice in all the ornamental arts, and 
would, at the lame time, become a popular inltrument for 
the purpofes of rational amufement. With thefe views I 
thought it advifableto fecure the exclulive property of it 
by a patent; but, in confequence of one of the paten t-inftru- 
ments having been exhibited to fome of the London opti¬ 
cians, fome of the remarkable properties of it became 
known, before any number of them could be prepared for 
fide. The fenfation excited in London by this premature 
exhibition of its effeCts is incapable of description, and can 
be conceived only by thole who witneffed it. It may be 
fufficient to remark, that, according to the computation of 
thole who were beft able to form an opinion on the fub- 
jeCt, no fewer than two hundred thoufand inllruments 
have been fold in London and Paris during three months ; 
[nor did the French fcruple to claim the invention for 
themfelves; for, not a month after the firft kaleidofcope 
had been received in London, and while the rage for this 
ingenious and fcientific toy was at its height, thofe made 
in Paris, precifely on the Englifli model, were expofed for 
fale in all the (hops under the name of “ kaleidofcopes, ou 
lunettes Francaifes.”] Out of this immenfe number there 
is perhaps not one thoufand conltrufited upon fcientific 
principles, and capable of giving any thing like a correfit 
idea of the power of the kaleidolcope ; andof the millions 
who have witneffed its effects, there is perhaps notan hun¬ 
dred who have any idea of the principles upon which it is 
conltrufited, who are capable ofdiltinguifning the fpurious 
from the real inltrument, or who have fufficient know¬ 
ledge of its principles for applying it to the numerous 
branches of the ufeful and ornamental arts.” 
Under thefe circumltances, Dr. Brewlter has thought it 
neceffary to draw up the (hort treatife we have quoted, for 
the purpofe of explaining, in a popular manner, the prin¬ 
ciples and conftrufition of the kaleidofcope ; of del'cribing 
the different forms in which it is fitted-up; of pointing 
out the various methods of ufing it as an inltrument of 
recreation; and of inftrufiting the artift how to employ it 
in the numerous branches of the ufeful and ornamental 
arts to which it is applicable. 
Introductory Remarks. —The principal parts of the ka¬ 
leidofcope are two reflefiting planes, made of glafs, metal, 
or any other reflecting fubftance, ground perfectly fiat, and 
highly po’.ifhed. When thefe two plates are pirt together 
at an angle of 6o°, or the fixth part of a circle, as Ihown 
in fig. 1. Plate XV. and the eye placed at the narrow 
end E, it will obferve the opening AOB multiplied fix 
times, and arranged round the centre O, as Ihown in fig. 2. 
In order to underftand how this effefit is produced, let 
11s take a fmall feftor of white paper of the lhape AOB, 
fig. 2. and, having laid it on a black ground, let the ex¬ 
tremity AO of one of the refleftors be placed upon the 
edge AO of the feftor. It is then obvious, that an image 
AO b of this white feCtor of paper will be formed behind 
the mirror AO, and will have the fame magnitude and 
the fame fituation behind the mirror as the feftor AOB 
had before it. In like manner, if we place the edge BO 
of the other refieftor upon the other fide BO of the paper 
feftor, a fimiiar image BOt will be formed behind it. The 
origin of three of the leftors feen round O is therefore 
explained: the firft, AOB, is the white-paper lector feen 
by direfit vifion ; the fecond, AO b, is an image of the firlt 
formed by one refieftion from the mirror AO ; and the 
third is another image of the firlt, formed by one refieftion 
from the other mirror BO. But it is well known, that 
the reflefted image of any objeft, when placed before an¬ 
other mirror, has an image of itfelf formed behind this 
mirror, in the very fame manner as if it were a new ob¬ 
jeft. Hence it follows, that the image AO b, being as it 
were a new objeft placed before the mirror BO, will have 
an image aOc of itfelf formed behind BO ; and for the 
fame reafon the image BOa will have an image bOd of it¬ 
felf formed behind the mirror AO ; and both thefe new 
images will occupy the fame pofition behind the mirrors 
as the other images did before the mirrors. 
A difficulty now prefents itfelf in accounting for the 
formation 
