CHAPTER III.

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.

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.)

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.

Figure 14 shows a form of McIntosh stereopticon lamp.

Figure 15 is a Kliegl lamp for open arc lamps.

Figure 16 is an Edison lamp used for motion picture work.

Figure 17 is a Kliegl lamp used for focusing purposes.

Figure 18 shows the Powers lamp.

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.

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.

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.

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.

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.

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.

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.

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.