PROJECTION.
_Setting and Adjustment of Carbons._--To project a picture upon a screen
properly is an art and requires close study and some knowledge of all
the factors involved. The most important factor is that of the light.
Electric light is so universally used at the present time that it is
hardly necessary to mention the other sources of illumination.
[Illustration: FIGURE 7.]
The electric current with which the operator has to deal may be either
alternating or direct, and the kind is of great importance. The color of
the light obtained from a direct-current arc is not only superior to
that obtained from an alternating-current arc but is obtained at a much
lower cost since, as we shall presently see, it is so much more
efficient.
To project clear white light upon the screen is impossible, some color
will always be in it. But by careful attention and by training himself
to notice slight degrees of color, the operator can learn to render a
light which will be clear enough to satisfy the majority of the
spectators. In order to obtain this light, the source from whence it
comes should be located exactly in the optical axis of the lens system;
that is, a straight line drawn through the center of all of the lenses
should pass also directly through the center of the arc as indicated in
Figure 7. (For comprehensive treatise on lenses, see Chapter XII.)
[Illustration: FIGURE 8.]
[Illustration: FIGURE 9.]
[Illustration: FIGURE 10.]
[Illustration: FIGURE 11.]
[Illustration: FIGURE 12.]
[Illustration: FIGURE 13.]
Most of the light, we have already seen, is emitted from the crater of
an arc of which there is but one in a direct-current arc and two in an
alternating current arc. In order to obtain the most light with the
least expenditure of current and heat in the lamp house, the crater must
be formed in such a manner as to face the center of the condensers as
nearly as possible. Since, however, there are always two electrodes and
the current must pass from one to the other, the crater always tends to
face the lower electrode if the upper one is positive. It is, therefore,
impossible to get the full benefit of the light for the condenser; we
must be satisfied with getting a part of it, and to do this such
settings of electrodes as are shown in Figures 8 to 13 are used. About
the relative merits of these various settings there is considerable
dispute and the best advice that can be given to any new comer in the
operating line is to make his own experiments and find out for himself.
The fact that a certain point is much disputed, alone indicates that
there is no exact knowledge available; for we very seldom have any
differences of opinion about the things that we can prove.
In the operating line very much depends upon the judgment of the
operator. Electrode setting like that of Figure 8 may be good for an
operator who is extremely careful and has a reliable machine which
requires a minimum of attention. But it can readily be seen that if the
top electrode were fed a trifle too far forward, the crater would form
underneath and the lenses would receive but a small part of the light.
Each of the settings given has its peculiarities and it is best for any
operator who has not done so, to try them all out and find which one
best suits him and his conditions.
Figures 8 to 10 show the settings used with direct-current arcs; while
those illustrated by Figures 11 to 13 are used with alternating-current
arcs.
With alternating-current arcs the problem is even more difficult than
with direct, for we have here two craters to deal with; and if we wish
to use the light from both, we shall have to be very careful about it.
If the electrodes are not set exactly right, we may get a double spot
and poor illumination at the center of the screen. Perhaps most
operators will soon give up the idea of using the light of both craters
and will settle down to an electrode setting something like that shown
in _A_, Figure 7. In this setting both electrodes are angled and the
lower one is set a little ahead of the upper. This has a tendency to
draw the crater of the upper electrode forward, thus improving the light
on the condenser; but if this be carried too far, the lower electrode
will obstruct the lower part of the lens. The lower electrode must
always be set so that it allows all parts of the condenser to receive
direct rays of light from the crater of the upper. The electrode must
align perfectly in the vertical plane as shown in _B_, Figure 7, or the
arc will move while burning.
In order to enable the operator to arrange his electrodes at any angle
and to bring them into the center of the optical system, arc lamps are
made up in various ways as illustrated in Figures 14 to 19. The simpler
types are used only in stage lighting lamps where the centering is not
so important. The more elaborate lamps are provided for motion picture
arc lamps and allow of all necessary adjustments which are: feed
electrodes; move lamps forward or back; up or down; sideways and angle
electrodes.
Where direct current is used, the upper electrode must be fed
approximately twice as fast as the lower; but with alternating current,
they both feed at practically the same rate.
[Illustration: FIGURE 14.]
Figure 14 shows a form of McIntosh stereopticon lamp.
[Illustration: FIGURE 15.]
Figure 15 is a Kliegl lamp for open arc lamps.
[Illustration: FIGURE 16.]
Figure 16 is an Edison lamp used for motion picture work.
[Illustration: FIGURE 17.]
Figure 17 is a Kliegl lamp used for focusing purposes.
[Illustration: FIGURE 18.]
Figure 18 shows the Powers lamp.
[Illustration: FIGURE 19.]
Figure 19 shows one of the Motiograph Company lamps.
_Optical System._--In Figure 20 we have the complete optical system of
the moving picture or stereopticon outfit. The crater of the arc lamp
and the center of the objective lens are at the conjugate focal points
(see Optics) and must always be in this relation. The size of the
picture projected upon the screen is governed entirely by the focal
length of the objective lens and the distance of the screen from this
lens. The shorter the focal length, the greater will be the bulging out
or rounding of the lens, and the larger the picture projected. The
objective lens is always fitted with an adjusting device of some kind by
which it can be moved forward or back a little to focus the picture
properly.
[Illustration: FIGURE 20.]
In order to project a picture properly, it is necessary that the center
of the arc or other illuminant, the center of the condensers, and the
center of the objective, all fall in one straight line as indicated in
Figure 20. The condensers are provided for the purpose of gathering and
condensing as many of the scattering light rays of the arc lamp as
possible and bringing them to bear upon the slide and the objective.
The light used must come either from a reasonably small source or from a
larger source far enough away so that the rays can be considered as
parallel. The focal point for parallel rays would, however, differ
somewhat from that of a point source and such illumination is seldom
used; in fact, it is used only where special arrangements are made for
it.
One of the principal points to be borne in mind in trying to project a
good clear picture is to keep the arc down to as small a point as is
practicable. A long arc can be tolerated only when it is absolutely
impossible to obtain sufficient illumination from a short arc; as, for
instance, in operating the Kinemacolor machines, in which from 80 to 100
amperes are used with a very long arc. The above expedient is imperative
because the colored discs through which the light must pass absorb a
great amount of it and the definition or outline of the picture is apt
to be poor.
The position of the arc with reference to the condensers is also an
important point to consider. The focal length of the condensers
determines the point at which the arc must be maintained. The flatter
the condensers are, the farther away the arc may be, and the less will
be the heating; but this position is accompanied with considerable loss
of light.
For the purpose of projection we can use only the light which strikes
the condensers direct from the arc. Rays reflected by the lamp house do
not pass through the condensers in the same direction as those coming
directly from the crater and will not focus with them. Hence, the
farther the arc is from the condensers, the smaller will be the
percentage of light used; the shorter the focal length of the
condensers, the closer to them must the arc be maintained, and the
greater will be the percentage of light used. But if the light is
brought too close, there will be undue heating of the condensers and
these, especially the one nearest the light, will be likely to break. So
great is the heat produced that sometimes the two lenses are partially
melted and welded together. This is a frequent occurrence in cases where
very heavy currents are used. It must be recalled that the heat produced
is proportional to the square of the current and that other things being
equal, 80 amperes would produce four times the heat of 40 amperes.
Condenser breakage is quite an important subject and one upon which
there is much argument among operators. Many of the theories held are,
however, not plausible enough to merit mention. The principal cause is
no doubt overheating without allowing sufficient room for expansion in
the setting. No lens should ever be set so that it does not move freely
even while it is hot. Even if free while cold, the expansion, where the
heating is great, may be sufficient to tighten it in the casing, and
this is likely to cause breakage. The best methods of preventing heating
are: a large lamp house well ventilated and condensers of such focal
length as to allow the arc to be maintained at some distance from them.
Drafts of air are often given as the cause of breakage, but the truth of
this is rather problematical. There is no doubt that sudden contraction,
due to rapid cooling, would have a strong tendency to break them; but
the air in operating rooms is not often cold and is not likely to
strike the lens anyway. It must be noted that it is usually the inner
lens, which is ordinarily enclosed, that breaks.
[Illustration: FIGURE 21.]
In the projection of moving pictures there are two important points that
must always be considered. (1) the size of the spot on the gate at which
the film appears, and (2) the clearness of the field or light on the
screen. By properly adjusting the arc, we can make the spot any size we
desire; and the smaller we make it, so long as it covers the whole
aperture, the brighter the light will be. But if we make this spot too
small, we shall bring in the fringe of color which always appears at the
outer edge. Color of this kind is objectionable and must be avoided as
much as possible; but it is not necessary to go to extremes. A little
coloring will not be noticed by the audience and will therefore not be
objectionable. With a given system there will thus be a certain size of
spot which gives the best results obtainable. Considering that if the
spot is increased in size, the light becomes clearer but also less
intense; and that if the spot is decreased in size, the light on the
screen, though more brilliant, is liable to show coloring, a good
operator should practice distinguishing the coloring and make himself as
proficient in this art as possible. The customary proportions of spot
and aperture are shown in Figure 21.
Coloring appears, however, from another cause also, viz., improper
centering or adjustment of the arc lamp with reference to the
condensers. If the arc is not properly adjusted, bands of color such as
are indicated in Figure 22 may appear in any of the positions shown.
This is commonly spoken of as the “ghost”, and it must be eliminated. It
is not possible to get rid of it entirely, but by a little skill,
patience, and experience, it can be reduced to a negligible amount. When
the spot is right and the screen clear, the picture may be focused by
adjusting the objective lens.
[Illustration: FIGURE 22.]
To focus sharply, it is advisable to move the lens in one direction
until the picture appears a trifle blurred; then move it in the opposite
direction until at this point there is also a blurred picture. The
exact focus will be at a point half way between the two. To focus the
lens in this manner is important where the slide or film has some play,
as when the aperture plate on a machine is worn and allows the film some
movement.
_Current Required._--The measurement of the candle power of arc lamps
has never been satisfactorily taken, and the difficulties encountered in
determining it for a projecting arc are especially great because only a
small part of the total light can be utilized and this is constantly
varying. The light may, however, be assumed as proportional to the
wattage of the arc, hence, we can best judge it by noting the volts and
amperes. Where a very strong light is desirable, the arc is usually
drawn out to some length; and as there is a rise in voltage, with a long
arc, in such a case, the light increases at a greater rate than the
amperage. In ordinary projection work, the arc is kept quite short
because of the better definition obtainable by the use of such an arc;
and we may assume that the light obtained is nearly directly
proportional to the amperage. This relation of light and the current
input to the lamp will be practically correct, especially if the size of
the electrodes chosen is proportional to the amperage.
_Current Required for Projecting._--The value of the current to be used
for projection is a matter of some dispute among operators and probably
much of this is caused by the absence of ammeters, most operators merely
guessing at what they are using, or being guided by markings of
rheostats or compensators. In most cases something like 40 amperes seems
to be the rule.
In order to give the reader a clear understanding of the theoretical
requirements, Table I has been prepared. This table is not intended to
act as an accurate guide, but merely to show the amperage theoretically
required with different sized pictures, to bring about the same
illumination in each case.
TABLE I.
CURRENT REQUIRED FOR DIFFERENT SIZE PICTURES.
-----------+--------------+------------------------
Greatest | | Amperes
Dimension | Area +----------+-------------
of Picture | Illuminated. | Direct | Alternating
in feet. | | Current. | Current.
-----------+--------------+----------+-------------
5 | 39 | 8 | 12
6 | 56 | 11 | 16
7 | 77 | 15 | 22
8 | 100 | 20 | 30
9 | 127 | 25 | 37
10 | 157 | 31 | 45
11 | 189 | 38 | 57
12 | 224 | 45 | 67
13 | 260 | 52 | 78
14 | 307 | 60 | 90
-----------+--------------+----------+-------------
Two errors are very common in the computation of the light intensity for
a given picture: (1) the length of throw governs the amperage; and (2)
the amperage depends upon the actual space to be illuminated. Apparently
only an oblong square of exactly the proportions of the aperture in the
machine is illuminated, but in reality the light must be spread out so
that its total illumination covers a circle enclosing the actual visible
picture. This is illustrated in Figure 23 where the enclosed oblong
square represents the space illuminated on the screen and the circle
represents the area over which the light must be spread. The portion
shown by shading is nearly equal to the clear portion and shows that
half of the light is wasted since it is blocked out by the cooling plate
in the machine or the framework of the slides. With increasing size of
picture, the light is, however, diminished in proportion to the area of
the circle and not in proportion to the area of the picture. If, for
instance, the picture were to retain its width and be reduced in height
by one half, or even more, there would still be about the same quantity
of illumination required. For this reason we have, in Table I, given
only the maximum dimension of the picture and have based the amperage
calculation upon the area of the circle which encloses the picture.
[Illustration: FIGURE 23.]
The values given are less than are generally used for small pictures and
more than are generally used for large pictures. As a rule much light is
wasted on small pictures because the apparatus is at hand to deliver it;
with large pictures, the illumination is often poor because transformers
and rheostats are seldom fitted to deliver more than 60 amperes. Much
light can easily be wasted if the picture is made too bright. In such a
case, much of the light is reflected back to the auditorium and this in
turn makes the picture appear less bright.
In determining the amperage necessary to show a picture properly, the
following conditions must be borne in mind, any one of which may
appreciably affect the result:
(1) _Nature of Screen._--A good screen will reflect more light than a
poor one.
(2) _Size of Picture._--The larger the picture, the more light will be
required.
(3) _Character of Film._--Some films are very dark and require extra
illumination.
(4) _House Illumination._--In some cities the law requires fairly bright
illumination of auditoriums and this makes the picture appear less
bright.
(5) _Atmosphere._--Where the air is full of dust, or where smoking is
allowed; much light will be absorbed.
(6) _Lenses._--Some lenses are badly discolored and absorb much light.
(7) _Electrodes and Electrode Setting._--This is a very important factor
and one which a good operator will never neglect.
_Selection of Lenses._--Upon the proper selection of lenses depends very
much the quality of the picture. The size of the picture, under given
circumstances, depends entirely upon the focal length of the objective.
With a given distance between lens and screen there is practically but
one size of picture obtainable. If we wish to obtain a picture of
another size by the use of the same lens, this can be done only by
sacrificing the definition and had better not be attempted.
Very large pictures are desirable only in large halls in which portions
of the audience are very far from the screen. Such a picture requires
very much light and, on account of its size, shows many imperfections to
those who sit in the front rows. It is better to limit the size of the
picture to one which can be easily illuminated, and thus avoid such
imperfections.
TABLE II.
MOTION PICTURE LENSES.
TABLE SHOWING SIZE OF SCREEN IMAGE WHEN MOVING-PICTURE FILMS ARE
PROJECTED.
Size of Mat opening 11-16 × 15-16 inch.
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
E.E. | 15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 60 | 70 | 80 | 90 |100
In. | ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
2-1/8| 4.8| 6.4| 8.0| 9.6|11.3|12.9|14.5|16.1| | | | |
| 6.5| 8.7|11.0|13.2|15.4|17.6|19.8|22.0| | | | |
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
2-1/2| | 5.4| 6.8| 8.2| 9.6|10.9|12.3|13.7|16.4| | | |
| | 7.4| 9.3|11.2|13.1|14.9|16.8|18.7|22.4| | | |
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
3 | | 4.5| 5.7| 6.8| 8.0| 9.1|10.3|11.4|13.7|16.0| | |
| | 6.2| 7.7| 9.3|10.9|12.4|14.0|15.6|18.7|21.8| | |
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
3-1/2| | | 4.9| 5.8| 6.8| 7.8| 8.8| 9.8|11.7|13.7|15.7| |
| | | 6.6| 8.0| 9.3|10.6|12.0|13.3|16.0|18.7|21.4| |
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
4 | | | 4.2| 5.1| 6.0| 6.8| 7.7| 8.5|10.3|12.0|13.7|15.4|
| | | 5.8| 7.0| 8.1| 9.3|10.5|11.6|14.0|16.3|18.7|21.0|
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
4-1/2| | | | 4.5| 5.3| 6.2| 6.8| 7.7| 9.1|10.6|12.2|13.7|15.4
| | | | 6.2| 7.2| 8.4| 9.3|10.5|12.4|14.5|16.6|18.7|21.0
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
5 | | | | | 4.8| 5.4| 6.1| 6.8| 8.2| 9.6|10.9|12.3|13.7
| | | | | 6.5| 7.4| 8.4| 9.3|11.2|13.0|14.9|16.8|18.7
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
5-1/2| | | | | 4.3| 4.9| 5.6| 6.2| 7.4| 8.7| 9.9|11.2|12.4
| | | | | 5.9| 6.7| 7.6| 8.4|10.2|11.9|13.6|15.3|17.0
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
6 | | | | | | 4.5| 5.1| 5.7| 6.8| 8.0| 9.1|10.3|11.4
| | | | | | 6.2| 7.0| 7.7| 9.3|10.9|12.4|14.0|15.6
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
6-1/2| | | | | | | 4.7| 5.2| 6.3| 7.3| 8.4| 9.6|10.6
| | | | | | | 6.4| 7.1| 8.6|10.0|11.4|13.0|14.5
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
7 | | | | | | | 4.4| 4.9| 5.8| 6.8| 7.8| 8.8| 9.8
| | | | | | | 6.0| 6.6| 8.0| 9.3|10.6|12.0|13.3
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
7-1/2| | | | | | | | 4.5| 5.4| 6.4| 7.3| 8.2| 9.1
| | | | | | | | 6.2| 7.4| 8.7|10.0|11.2|12.3
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
8 | | | | | | | | | 5.1| 6.0| 6.8| 7.7| 8.5
| | | | | | | | | 7.0| 8.1| 9.3|10.5|11.6
-----+----+----+----+----+----+----+----+----+----+----+----+----+----
=Example=: With a lens of 5-1/2 inch focus at a distance of 35 ft. the
screen image will be 4.3×5.9; at 40 ft., 4.9×6.7; at 45 ft., 5.6×7.6;
etc.
=Note=: When ordering lenses, give size of picture wanted, and distance
from machine to screen.
TABLE III.
STEREOPTICON LENSES.
TABLE SHOWING SIZE OF SCREEN IMAGE WHEN LANTERN SLIDES ARE PROJECTED.
Size of Mat opening 2-3/4 × 3 inches.
------+---+----+----+----+----+----+----+----+----+----+----+----+----
E.F. |15 | 20 | 25 | 30 | 35 | 40 | 45 | 50 | 60 | 70 | 80 | 90 |100
In. |ft.| ft.| ft.| ft | ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.| ft.
------+---+----+----+----+----+----+----+----+----+----+----+----+----
5 |8.0|10.8|13.5|16.3|19.0| | | | | | | |
|8.8|11.8|14.8|17.8|20.8| | | | | | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
5-1/2|7.3| 9.8|12.3|14.8|17.3|19.8| | | | | | |
|7.9|10.7|13.4|16.1|18.8|21.6| | | | | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
6 |6.6| 8.9|11.2|13.5|15.8|18.1|20.4| | | | | |
|7.3| 9.8|12.3|14.8|17.3|19.8|22.3| | | | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
6-1/2|6.1| 8.2|10.4|12.5|14.6|16.7|18.8| | | | | |
|6.7| 9.0|11.3|13.6|15.9|18.2|20.5| | | | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
7 |5.7| 7.6| 9.6|11.6|13.5|15.5|17.5|19.4| | | | |
|6.2| 8.3|10.5|12.6|14.8|16.9|19.0|21.2| | | | |
------+---+----+----+----+----+----+----+----+----+---+----+----+----
7-1/2|5.3| 7.1| 8.9|10.8|12.6|14.4|16.3|18.1| | | | |
|5.8| 7.8| 9.8|11.8|13.8|15.8|17.8|19.8| | | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
8 | | 6.6| 8.4|10.1|11.8|13.5|15.2|17.0|20.4| | | |
| | 7.3| 9.1|11.0|12.9|14.8|16.6|18.5|22.3| | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
8-1/2| | 6.2| 7.9| 9.5|11.1|12.7|14.3|16.0|19.2| | | |
| | 6.8| 8.6|10.3|12.1|13.9|15.6|17.4|20.9| | | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
9 | | 5.9| 7.4| 8.9|10.5|12.0|13.5|15.1|18.1|21.1| | |
| | 6.4| 8.1| 9.8|11.4|13.1|14.8|16.4|19.8|23.1| | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
9-1/2| | 5.6| 7.0| 8.5| 9.9|11.4|12.8|14.2|17.1|20.0| | |
| | 6.1| 7.6| 9.2|10.8|12.4|14.0|15.5|18.7|21.9| | |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
10 | | 5.3| 6.6| 8.0| 9.4|10.8|12.2|13.5|16.3|19.0|21.8| |
| | 5.8| 7.3| 8.8|10.3|11.8|13.3|14.8|17.8|20.8|23.8| |
------+---+----+----+----+----+----+----+----+----+----+----+----+----
12 | | | 5.5| 6.6| 7.8| 8.9|10.1|11.2|13.5|15.8|18.1|20.4|
| | | 6.0| 7.3| 8.5| 9.8|11.0|12.3|14.8|17.3|19.8|22.3|
------+---+----+----+----+----+----+----+----+----+----+----+----+----
14 | | | | 5.6| 6.6| 7.6| 8.6| 9.6|11.6|13.5|15.5|17.5|19.4
| | | | 6.2| 7.3| 8.3| 9.4|10.5|12.6|14.8|16.9|19.0|21.2
------+---+----+----+----+----+----+----+----+----+----+----+----+----
16 | | | | | 5.8| 6.6| 7.5| 8.4|10.1|11.8|12.5|15.2|17.0
| | | | | 6.3| 7.3| 8.2| 9.1|11.0|12.9|14.8|16.6|18.5
------+---+----+----+----+----+----+----+----+----+----+----+----+----
18 | | | | | 5.1| 5.9| 6.6| 7.4| 8.9|10.5|12.0|13.5|15.1
| | | | | 5.6| 6.4| 7.3| 8.1| 9.8|11.4|13.1|14.8|16.4
------+---+----+----+----+----+----+----+----+----+----+----+----+----
20 | | | | | | 5.3| 6.0| 6.6| 8.0| 9.4|10.8|12.2|13.5
| | | | | | 5.8| 6.5| 7.3| 8.8|10.3|11.8|13.3|14.8
------+---+----+----+----+----+----+----+----+----+----+----+----+----
22 | | | | | | | 5.4| 6.0| 7.3| 8.5| 9.8|11.0|12.3
| | | | | | | 5.9| 6.6| 7.9| 9.3|10.7|12.0|13.4
------+---+----+----+----+----+----+----+----+----+----+----+----+----
24 | | | | | | | | 5.5| 6.6| 7.8| 8.9|10.1|11.2
| | | | | | | | 6.0| 7.3| 8.5| 9.8|11.0|12.3
------+---+----+----+----+----+----+----+----+----+----+----+----+----
=Example=: With lens of 10-inch focus at a distance of 20 ft. the screen
image will be 5.3×5.8; at 25 ft., 6.6×7.3; at 30 ft., 8.0×8.8; at 50
ft., 13.5×14.8.; etc.
Table II shows the size of picture obtainable from films, and Table III,
the size obtainable from lantern slides. Since the slide pictures must
be shown upon the same screen as the film, it can be seen from the
tables that lenses of different focal length must be used for the two.
The aim should be to get the two pictures to match as nearly as
possible, but as they are not of the same proportions, it is impossible
to match them exactly in both directions. The nearest approximation that
can be brought about by standard lenses is illustrated in Figure 24. The
heavy lines show the dimension of the picture projected through the
film, and the light and dotted lines show the dimensions obtainable by
the use of slides. If the slide picture is matched to the height of the
film, it will be considerably narrower; if it is matched to the sides,
it will be considerably higher. It would of course be possible to trim
down slides so that the dimensions of the two pictures would be exactly
alike; but as most all stereopticon slides belong to traveling actors
this is not practicable.
[Illustration: FIGURE 24.]
If the focal length of a lens is not known, it can easily be measured by
focusing some distant object, an incandescent lamp for instance, against
the wall of a room or against some screen placed upon a table as shown
in Figure 25. In the case of a single plano-convex lens, the
measurements must be made from both sides--first one side turned toward
the light, and then the other. There will always be some difference
between the two measurements and we must take the mean of the two. To
get the measurement accurately, place a rule upon a table and set up
some suitable object upon which the picture can be projected. Turn the
flat side of the lens toward the screen and focus some distant object by
moving the lens to a point at which the object selected will appear
clearly upon the screen. Note the distance of the flat side of the lens
from the picture. Now turn the lens half way around and focus again in
the same manner, noting this distance also. Add the two measurements and
divide by two; this will give the focal length of the lens. In the case
of an objective lens, we must turn the side which bulges out most toward
the screen and focus in the same manner.
[Illustration: FIGURE 25.]
With the objective lens we have two possible focal lengths to consider.
If we measure from the center of the lens to the screen, we shall obtain
what is called the _equivalent focal length_ (usually abbreviated E.F.
or e.f.). If, instead, we take measurements from the face of the lens
nearest the screen, we shall obtain what is termed the _back focus_, or
b.f., of the lens. In all cases it is important, when ordering, to state
which of the two is meant.
Lenses may also be tested for chromatic and spherical aberration.
Chromatic aberration is the fault of showing colors unduly. It is
impossible to avoid a fringe of color when using only a single lens, but
where we have a complete optical system, consisting of two condensers
and an objective, it must be possible to adjust the combination so that
practically no color is visible. Spherical aberration is best tested for
by laying out very accurately, as in Figure 26, a set of small squares
upon some material that will not be damaged by the heat of the
lamp--mica for instance--and projecting this upon the screen. If the
lenses are all good, the lines will all appear square; if the lenses are
poor, the lines will appear curved a little, or perhaps considerably.
[Illustration: FIGURE 26.]
The diameter of the ordinary condenser lens is 4-1/2 inches. Smaller
lenses than this cannot well be used because they would not cover the
diagonal of lantern slides. A very common focal length of condenser is
6-1/2 inches. There is no very accurate relation necessary between the
focal length of condenser and objective. There is considerable
difference of opinion on this subject and much of it is induced by the
possibility of condenser breakage which is increased by using
condensers of short focal length, but in this case, as in many others,
the operator must find out by his own experiments.
A very good plan--since, on account of breakage, extra lenses must be
carried anyway--is to carry two 7-1/2-inch and two 6-1/2-inch condensers
and experiment with these. The two of the same diameters may be tried
together and also those of different focal lengths, using the one of
shorter focal length either in front of or behind the other.
HINTS ON MANAGEMENT OF PROJECTING ARCS.
Before starting to work about the lamp, be sure the switch is off.
See that the lamp house is clean and spark tight.
The gauze provided at the top must be kept free from dirt and carbon
ash, or the house itself may get too hot.
The house should be of such dimensions, relative to the length of
electrodes used, that the latter cannot touch either at the top or
bottom and thus ground the circuit on the lamp house and possibly burn a
hole in it.
See that your lamp mechanism is well aligned so that electrodes center
at all positions.
All of the screws and adjustments should be well lubricated frequently.
The heat in the lamp house soon evaporates all lubrication.
Where lamps are used much and carry heavy currents, the leading in wires
will probably need reconnecting about once a week. It is best to
reconnect them some time before they burn off rather than be obliged to
do this during a show.
See that your polarity is right. With direct current, the upper
electrode will retain its heat longer than the lower if connections are
made properly. With alternating current the polarity is immaterial.
Always point your electrodes, especially the lower. If the lower
electrode is not pointed, it will interfere with the light of the
crater.
The recommendations for sizes of upper and lower electrodes vary
somewhat but run mostly to 5/8 inch for upper and 1/2 inch for lower.
The size depends very much upon the current used. If the electrodes are
too large, the arc will travel around the outside and yield a poor and
uneven light.
Always use cored carbons for alternating current.
The best length for electrodes is about 6 inches, if they do not strike
the lamp house.
Notch the carbon electrode a little before attempting to break it off.
Many operators are in the habit of watching the arc, opening the
lamp-house door to look at it. Not only is this injurious to the eyes
but it also interferes with proper judgment of the illumination of the
picture. A better way is to punch a very small hole in the lamp house
exactly opposite the arc. Over this opening a lens may be placed, and a
picture of the arc may be thrown against a wall or screen set up for
that purpose. A picture of the arc is also obtainable in another way: If
the lamp is once set exactly right, a cross may be painted at the proper
place on the screen which will indicate the exact point where the arc
should be maintained. The arc will of course appear inverted. Another
method of keeping the arc always in view without inconvenience consists
in arranging a small mirror, at an angle to the peep glass in the door,
so that it will reflect the arc towards the operator.
An adjustable resistance should always be kept in reach so that the
current may be varied to suit different films or stereopticon lamps.
Keep your hands as free from carbon dust as possible. This dust is
responsible for much damage to films.