OPTICS. 
focal length fhould be ufed ; and, when the objeCts are 
diftant, and not very large, a lens, whofe principal focal 
length is nearly equal to the greateft diftance of the lens 
from the refleCtors, thould be employed. 
When this compound kaleidofcope is ufed as a fimple 
inftrument for viewing objects held clofe to the aperture, 
the tube AB is pufhed in as far as it will go, the cell with 
the object-plate is flipped upon the end C of the outer 
tube, and the inltrument is ufed in the fame way as. the 
fimple kaleidofcope. In applying the compound kaleido¬ 
fcope to diftant objects, the cell is removed, and the lens 
fubftituted in its place. The inftrument is then directed 
to the objeCt, and the tube AB drawn out till the pat¬ 
tern appears perfedtly fymmetrical. When this is done, 
the pattern is varied, both by turning the inftrument 
round its axis, and by moving it in any direction over 
the objedl to which It is applied. When the object is 
about four inches from the lens, the tube requires to be 
pulled out as far as poflible, and for greater diftances it 
muft be pufhed in. The points fuited to different dif¬ 
tances can eafily be determined by experiment, and marked 
on the inner tube, if it fhould be found convenient. In 
molt of the inftruments there is, near the middle of the 
tube AB, a mark which is nearly fuited to all diftances 
beyond three feet. The objedf-plates held in the hand, 
or the mirror-box placed upon a table, at a diftance 
greater than five or fix inches, may be alfo ufed when the 
lens L is in the tube. The furniture of a room, books 
and papers lying on a table, pidlureS on the wall, a blazing 
fire, the moving branches and foliage of trees and flirubs, 
bunches of flowers, horfes and cattle in a park, carriages 
in motion, the currents of a river, moving infefls, the 
fun fhining through clouds or trees, and, in fliort, every 
objeCt in nature, may be introduced by the aid of the 
lens into the figures created by the inftrument. 
The patterns which are thus prefented to the eye, are 
efl'entially different from thofe exhibited by the fimple 
kaleidofcope. Here the objects are independent of the 
obferver, and all their movements are reprefented with 
the moft Angular erfeCt in the fymmetrical picture, which 
is as much fuperior to what is given by the fimple inftru¬ 
ment as the fight of living or moving objeCts is fuperior to 
an imperfeCt portrait of them. When the flame of a 
blazing fire is the objeCt, the kaleidofcope creates from 
it the moft magical fireworks, in which the currents of 
flame which compofe the picture can be turned into every 
poflible direction. 
In order to mark with accuracy the points on the tube 
AB, fuited to different diftances, the inftrument fhould 
be directed to a ftraight line, inclined, like mn, fig. 3, to 
the line bifeCting the angular aperture AOB, and brought 
nearer to the centre O of the field. The perfect junction 
of the reflected images of the line at the points ml, n', &c. 
fo as to form a ftar, or a polygon with falient and re-en¬ 
tering angles, will indicate, with great nicety, that the 
tube has been pulled out the proper length for the given 
diftance. In this way a fcale for different diftances, and 
fcalesfor different lenfes, may be marked on the tube. 
In the conftruCtion of the compound kaleidofcope, the 
greateft care mufl; be taken to have the lens of fufficient 
magnitude. If it be too final], the field of view embraced 
by the inftrument will not coincide with the circular pat¬ 
tern ; that is, the centre of the circular pattern will not 
coincide with the centre of the field; and this eccentri¬ 
city vrill increafe as the diftance of the lens from the re¬ 
flectors is increafed, or as the objeCt introduced into the 
picture approaches to the inftrument. The boundary of 
the luminous field is alfo an irregular outline, confifting 
of difunited curves. Thefe irregularities are eafily ex¬ 
plained. "When the lens is too fmall, the luminous 
field is bounded by the brafs rim in which the glafs is 
fixed ; and, as this brafs rim is at a.diftance from the re¬ 
flectors, the portion of it prefented to the angular aper¬ 
ture cannot be formed byfucceflive reflexions into a con- 
VOL. XVII. No. 1205, 
049 
tinuous curve ; and, for the fame reafon, the upper feCtors 
of the luminous field are larger than the lower ones, and 
confequently the centre of the pattern cannot coincide 
with that of the field. In order to avoid thefe defeCts, 
therefore, the diameter of the lens fhould be fuch, that, 
when it is at its greateft diftance from the reflectors, the 
field of view may be bounded by the arch AB, fig. n, 
and not by the brafs rim which holds the lens. This 
may be readily known by removing the eye-glafs, and 
applying the eye at E when the lens is at its greateft dif¬ 
tance. If the eye cannot fee the brafs rim, then the lens 
is fufficienrly large ; but, if the brafs rim is vifible, the 
lens is too fmall, and muft be enlarged till it ceafes to 
become vifible. Sometimes the lens has been made fo 
fmall, that the brafs rim is feen not only at AB, but ap¬ 
pears alfo above the angular point O, and produces a 
dark fpot in the centre of the picture. 
Inftead of uling two tubes, a lens is fometimes fitted 
into a tube about an inch longer than the focal length ofi 
the glafs, and this tube is flipped upon the object-end 
ABO, fig. 11. This mode of applying the lens is, how¬ 
ever, inferior to the firft method, as there is little room 
for adjufting it to different diftances ; whereas with the 
long tube all objeCts at a greater diftance than four inches 
from the lens may be introduced into the picture; a pro¬ 
perty which poflefles very peculiar advantages. 
The extenfion of the inftrument to diftant objects is 
not the only advantage which is derived from the ufe of 
the lens. As the pofition for giving perfect fymmetry is 
rather within the extremities of the refleCtors than with¬ 
out them, and as it is impoflible to place movable ob¬ 
jects within the reflectors, we are compelled to admit a 
fmall error, arifing principally from the thicknefs of the 
objeCts, and from the thicknefs of the plate of glafs 
which is neceflarily interpofed between the objeCts and 
the refleCtors. The compound kaleidofcope, however, 
is entirely free from this defeCl. The image of a diftant, 
or even of a near, objeCt can be formed within the reflec¬ 
tors, and in the mathematical pofition of fymmetry; 
while, at the fame time, the fubftitution of the image for 
the objeCt itfelf, enables us to produce all the changes in 
the picture which the motion of the objeCt could have ef¬ 
fected, merely by turning the inftrument round its axis, 
or by moving it horizontally, or in any other direction 
acrofs the objeCt. 
Pohjangular Kaleidofcope .—In all the preceding inftru¬ 
ments, the reflecting planes are fixed at an invariable 
angle, which is fome even aliquot part of 360°; and 
therefore, though the forms or patterns which'they create 
are literally infinite in number, yet they have all the 
fame character, in fo far as they are compofed of as many 
pairs of direCt and inverted images as half the number of 
times that the inclination of the refleCtors is contained in 
360°. It is therefore of the greateft importance, in the 
application of the kaleidofcope to the arts, to have it 
conftruCted in fuch a manner, that patterns compofed of 
any number of pairs of direCt and inverted images may 
be created and drawn. With this view, the inftrument 
may be fitted up in various ways, with paper, cloth, and 
metallic joints, by means of which the angle can be varied 
at pleafure; but the moft convenient method is fliown 
in the figures 15, 16, and 17. which reprefent the polyan- 
gular kaleidofcope with metallic refleCtors, as made by- 
Mr. R. B. Bate, optician, in the Poultry, who has devoted 
much time and attention to the perfecting of this fpecies 
of kaleidofcope. Fig. 15 fliows the complete inftrument, 
mounted upon its Hand ST, with its objeCt-glafs at CD, 
and its eye-glafs feparate at E. The fame letters denote 
the fame parts in each of the three figures. Fig. 16 is a fec- 
tion of the fame inftrument in the direction of its length; 
and fig 17 is a tranfverfe feCtion of it through the line 
ST, fig. 16. The tube of this inftrument is compofed of 
two cones, MM, NN, fig. 15, 16, connected together by 
a middle piece or ring RR, into which they are both 
8 C fcrewed a 
