LXXXIII
are replaced by shades and inversely. The unaffected Iodide
having been removed from the positive picture (else the whole
picture would become black when exposed to the action of light)
the operation of photographing is complete. Whether it will
be ever possible to fix also the various colours of objects and to
point with light just as we now draw with it, is a question dif
ficult to decide. What we want for that, would be a substance
susceptible in such a manner as to re-produce the colours
of the sun's spectrum (see § 224). We do not say that this is
impossible.
§ 202 3) Lanterna magica, Magic Lantern (invented
by the German Jesuit Kischner) projects on a wall in a darkroom
a magnified image of a small object painted on glass. The rays
of a lamp standing in the focus of a concave mirror are con
tracted by a lens upon the object, a magnified real image of
which is produced on the wall. 394.
§ 203. 4) Telescopes. As to the general construction of
these instruments, used for the observation of distant objects,
there are two lenses in it: one called the objective for concen
trating the light from the object in a focus and the other the
eyepiece (ocular), which is near the eye for magnifying. The
invention was quite an accidental one.
§ 204. There are the following kinds: 1) The Astronomical
telescope (invented by Kepler 1630) which gives inverted
images (does not matter for the observation of stars). By put
ting a third lens between the objective and ocular we see the
objects in their proper position, and this instrument is called
“Terrestrial Telescope”. 2) Galileo's Telescope, in which the
object-glass is a double convex and the ocular a double concave
lens (opera-glasses constructed on this principle). It is used
only for a small field of view. 3) Reflecting Telescope (or
F*
