ON THE CONSTRUCTION OF MICROSCOPIC KALEIDOSCOPES.
The name of _Microscopic Kaleidoscope_ may be given to the instrument,
under two forms, namely, when it is made to produce symmetrical
patterns from microscopic objects, or when it is made so short that a
lens of a high power is necessary at one end of the reflectors, to see
distinctly, and magnify the objects at the other end. In both these
forms I have often constructed them so small as one inch and one inch
and a half in length. As the Kaleidoscope, in this minute state, has
been applied both in this country and abroad, as a female ornament, we
shall proceed to point out the best method of constructing it.
Since the aberration from symmetry increases, as the length of the
reflectors is diminished, and since the light of the field diminishes
from the same cause, it becomes extremely difficult to obtain correct
figures, and uniformity of light in small instruments. In order to
overcome these difficulties, as far as possible, the reflectors should
be metallic, and may be either made of polished steel or polished
speculum metal. The inclination at which they are fixed should not be
less than 36° or 45°; and the eye-hole, which should not exceed ¹/₁₅th
of an inch in diameter, must be placed as near as possible to the
angular point. Since the aberration from symmetry increases with the
distance of the object from the reflectors, and is much augmented in
small instruments, the greatest care must be taken to have the objects
at the least possible distance from the reflectors. To accomplish this,
the objects themselves should be as thin and slender as they can be
made; the colours should be brilliant and not gloomy; and they should
be separated from the reflectors by a thin film of the most transparent
mica, which is superior to glass of equal thickness, even if it could
be got, from its extreme toughness and elasticity. The mica, indeed,
is easily scratched, but if this should take place to any extent, it
can easily be replaced by a new film. It would even be of consequence
to bend the mica into a slight concavity, so as to permit the objects
to lie rather within than without the extremity of the reflectors. In
order to see the pattern with perfect distinctness, a lens must be
placed at the end of the instrument; the focal length of this lens,
however, must not be exactly equal to the distance of the objects from
the eye, but as much greater as possible, so that the eye, by a little
exertion, may be able to obtain distinct vision. The reason of this
will be understood, by considering that the images of the objects,
seen by reflexions, are thrown to a greater distance, as it were, from
the eye, and could not therefore be seen distinctly by using a lens
adjusted exactly to the nearest part of the picture. Consequently,
the focal length of the lens must be a mean between the distances of
different parts of the picture, that is, a little greater than that
which is suited to the sector seen by direct vision.
When small Kaleidoscopes are made with four or with three mirrors, the
preceding directions are equally applicable, the greatest care being
taken that the reflectors taper nearly to a point at the eye-end, so
as not to leave an aperture greater than ⅟₁₅th of an inch in diameter.
When they are made of solid glass, the focal length of the lens must be
determined from the principles contained in the last chapter.
The preceding instruments may be fitted up with a draw-tube and lens,
and when it is required to introduce drawings of pictures and statues,
small microscopic photographs may be employed.