a2 PHYSICS. 
them may exhibit many images. In pi. 21, fig. 9, let VW and ZW be two 
plane mirrors, at right angles to each other, with a luminous point placed 
between them. An eye at O sees, besides the point or object itself, its two 
images, a. a’, reflected from the two mirrors. But the rays reflected from 
one mirror are partly reflected back again by the other, on which account 
the images, a, a’, may themselves be considered as objects or radiant 
points: the two will form a third image in the same point, a’; more than 
these three images cannot exist at this angle. The number of images 
always depends upon the inclination of the mirrors ; if this amount to 60°, 
there will be six images, including the object, &c.; and, in general, this 
number (including the object) will be represented by oa where o is the 
a 
angle of inclination of the two mirrors. The number therefore increases 
with a diminution of the angle; when this is zero, or the mirrors become 
parallel, this number becomes infinite. 
Upon this principle depends the construction of the instrument invented 
by Brewster in 1817, and called by him the Kaleidoscope ( figs. 105, 106). 
This consists of a cylindrical or conical tube with a cap at one end, in 
which is a hole to look through. In the tube two plane mirrors are fixed, 
so as to form with each other a certain angle, 60° for instance. Instead 
of the mirrors usually employed, glass plates blackened on the back may be 
used. A false bottom of glass is placed at a short distance from the 
extremity opposite to that im which the eye-hole is situated, and over the 
"extreme end is fitted a second plate of glass by means of a cap. Pieces of 
colored glass, feathers, and other brightly colored objects are placed in the 
space between the two plates just mentioned. On looking through the 
eye-hole, towards the light, various hexagonal symmetrical images will be 
formed by the reflection of the objects in the -mirrors, which will be 
changed by every change in the relative position of the objects. Other 
polygonal images besides the hexagonal will be formed by varying the 
inclination of the mirrors. It must not be forgotten, however, that by 
too frequent reflections the light is enfeebled, and part of the image will be 
very faint. The dodecagon should be the maximum, in which case the 
angle of inclination must be 30°. The kaleidoscope is of great use in 
drawing patterns for various fabrics, for which purpose it has been 
variously modified, so as to produce other figures than the rosette. 
Wollaston’s reflecting Goniometer depends for its principle upon the 
reflecting of a ray of light. A goniometer is any instrument used for 
measuring the angle formed by any two sides of a crystal, and may be of 
various constructions, some of which will be found elsewhere. With 
regard to the goniometer of Dr. Wollaston, let (pl. 21, fig. 10) abcd be the 
section of a crystal, ab and ac the surfaces, appearing here as lines whose 
angle is to be measured. If now the edge, a (projected into a point in the 
figure), be horizontal, an eye at o observes in the surface, ab, the image of 
a distant horizontal line, to which the edge, a, is parallel also as a 
horizontal line. Let the eye be held in sucha manner that the image of 
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