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62 MODERN MAGIC LANTERNS.
planted by the form shown in B, in which two lenses are
employed, both with one convex and one plain surface, the
convex sides facing one another, and almost in contact. This
is an excellent form of condenser for all ordinary purposes,
if well made, and is met with far more often than any other
pattern. At c is another very good condenser known as
Herschel's, which is also often fitted to lanterns, and answers
in practice as well as B ; it is often made with the meniscus
or concavo-convex lens a little smaller than the other, and the
double convex lens has its inner curve flattened, in which form
it is known as Gravett's. n is one of the forms suggested
for a triple condenser, or one made of three lenses ; we lately
saw a lantern fitted with a triple condenser with a very
satisfactory result.
It will be seen from Fig. 52 that the amount of light
which falls upon a condenser, and which may be roughly
considered as the
amount available for
illuminating the slide,
depends upon the
diameter of the condenser
and the distance
at which it must be
from the lamp, for it
is obvious that a condenser
at A would receive
more light than
Fig. 52. one of the same size at
DIAGRAM TO SHOW THAT THE NEARER THE
CONDENSER IS TO THE SOURCE OF LIGHT,
THE MORE LIGHT IT RECEIVES.
B, since in the first case
not only will the rays
EE F F fall upon it which
fell upon B, but it will embrace G G H H. Certain practical
considerations, particularly the danger of having the thick
glass of a lens too near the light and consequent heat,
prevent the general use of condensers of such a size and
focus as to make the angle at c more than about 70 degrees.
For the usual photographic slide with a circular mask a
condenser of 4 inches diameter will be found large enough,
but if slides with very large masks with square corners are
to be shown, such a size will not be found sufficient, and
THE OPTICAL SYSTEM.
a 41 inch will be wanted. In getting a condenser, two
points should have attention : the glasses should be mounted
quite loosely in their brass rings so as to rattle when shaken,
if not, when expanded with the heat of the lamp they will
most likely break, and the lenses
63
themselves should be as
thin as possible at their edges. The slide should be as near
to the condenser as the carrier will allow. For photographic
enlarging, condensers will be required of a size dependent
upon that of the negative from which the enlargement is to
be made ; for good illumination its diameter should be at least
an inch longer than the diagonal of the negative.
The objective for projection purposes is, in the cheapest
lanterns, what is called a piano-convex (A, Fig. 53), or a
meniscus (B and c), but these are not at all to be desired,
Fig. 53. LANTERN OBJ...CTIVES.
especially in those of short focus ; foranything over 10 inches
they are frequently very satisfactory. With the introduction
of the rapid modern dry-plate in photography, however, the
necessity for the very " quick " lenses for portraiture has
largely disappeared, and portrait lenses as used by photographers
answer all the requirements of the average lanternist
admirably, and are frequently to be met with at a very low
price. This lens, shown diagrammatically in Fig. 53, D, was
the invention of Professor Petzval; and it or its modifications
in the hands of Dallmeyer are excellent for projection. Several
opticians now make special lantern lenses, based more or less
upon this pattern, one or two of which we have used at one
time or another with satisfaction. This type not only allows
a large amount of light to pass, but it also possesses a very
flat field, a quality the nature of which must be explained.