Optical Projection Part 8

While dealing with 'Special' lantern bodies, we should perhaps just mention here the numerous pattern lanterns made for the demonstration both of lantern slides and of Scientific Phenomena, such as the projection of insect life or other microscopic objects, polarised light experiments, electrical apparatus, opaque objects, &c. A detailed description of these lanterns and how to use them belongs to the second part of this work, as also does the popular cinematograph; but educational inst.i.tutes, and even boys' clubs, when considering the purchase of a lantern, might well reflect whether it would be advisable to spend a little more money in the acquisition of an instrument which can be utilised for a variety of purposes.

CHAPTER IX

LANTERN BOXES, STANDS, READING LAMPS, ETC.

Having now discussed all the essential parts of a lantern, the next points to be considered are those of lantern boxes and stands. It is best to take these together, as more often than not a lantern is arranged to stand upon its box during use, and the plan is both convenient and simple. The whole question is one to be settled upon its own merits in each individual case.

Sometimes neither box nor stand is wanted at all. The lantern is put away into a locked-up cupboard or other safe place, and used upon a permanent support or (as is often the case in a church) from a gallery at the back.

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In most cases, however, a box of some sort is desirable, and the two main considerations are strength and simplicity.

All patent arrangements, such, for example, as those in which the sides of the box fall down and provide trays for the slides, are beautiful in theory, but cannot be recommended in practice. A good, simple and substantial box is what is required, preferably painted black, and provided with strong handles.

One addition may be permitted, viz. a tilting top. Some means for tilting the lantern is always advisable, as it is seldom convenient to raise the instrument to the level of the centre of the screen, and a slight upward elevation does not appreciably distort the image. This arrangement for tilting may be either embodied in the lantern itself, as for instance in the instrument shown in Fig. 42, or may be provided for on the box or on the stand, if a stand is used.

It is, perhaps, an elaboration that may be regarded as not strictly necessary, as a book or two or other article may be placed under the lantern base as required; but a tilting arrangement is so convenient that it can be strongly recommended, and the addition is not expensive.

For large, long-range lanterns a strong deal box, on which the lantern can stand, is usually all that it is desirable to purchase in the way of a support. A good solid table can usually be found, which will do all the rest, as it must be remembered that a slight tilt at a long range means a good deal of total elevation.

Where this is not procurable a stand must be provided, and this for a large lantern should be strong and rigid. Anything in the way of a collapsible tripod should be avoided, but such an arrangement as Fig. 46 is quite good and rigid enough for all practical purposes.

For a _small_ lantern a tripod stand is quite suitable, though care must be taken that one of the legs does not get kicked, either by accident or design, or the result may be a catastrophe. {78}

_Slide Boxes._--On this subject not much need be said. The variety of patterns on the market is endless, some being designed from the point of view of safe transit by post, others for convenience of storage and cla.s.sification. It is essentially a case where each individual user must use his or her taste, and in any case the question of the box is one for the owner of the slides rather than for the lanternist.

[Ill.u.s.tration: FIG. 46.--Quadruple Lantern Stand.]

READING-DESKS, LAMPS, AND SIGNALS.--Some form of reading lamp for the lecturer is generally considered to be part of a lanternist's equipment, and the most usual pattern is fitted with a candle, after the manner of a carriage lamp, or else constructed to burn colza or other vegetable oil, such as supplied for cycle lamps. Oil gives the brighter light, but is apt to get spilled in transit, hence a candle lamp is the more {79} convenient for a travelling lecturer, while oil is to be preferred if transport is not a factor to be considered.

These lamps are usually constructed with a red flashing signal at the rear, actuated by a simple lever, by which the lecturer can communicate his wish for a change of slide, &c., to the lanternist (Fig. 47).

[Ill.u.s.tration: FIG. 47.--Reading Lamp.]

There are various other devices used for the same purpose, such as a castanet, to be held in the lecturer's hand and clicked when necessary, an electric bell to ring in the lantern box, &c. If this latter is used it is usual to remove the gong, the buzz of the hammer being sufficiently loud without it. Some lecturers again prefer to use no such apparatus at all, but simply to say 'Next slide' as required, or to tap on the floor with a pointer, and the choice of a suitable means of communication between lecturer and lanternist must be largely a matter of individual selection.

More elaborate _reading-desks_ are also supplied by most makers, but here again judgment must largely come into play in what is hardly a technical matter.

CHAPTER X

SCREENS AND SCREEN STANDS

The best of all screens for lantern purposes is undoubtedly a smooth whitewashed wall, and this is now provided in many halls where lantern exhibitions are usual. In places where this is not practicable the next best subst.i.tute is a canvas {80} screen, which rolls up and down (Fig. 48).

This can be obtained from any good maker, but again can only really be used as a _fixture_ in the hall where the lantern is to be used. It can, however, be fitted into a wooden box which can be painted or varnished to suit the other architecture, and the provision of such a screen is to be strongly recommended whenever possible. If portability is required, a linen or calico sheet that can be folded up is necessary, but this can never be hung absolutely flat, and also loses a considerable amount of light by transmission.

[Ill.u.s.tration: FIG. 48.--Roller Screen.]

A so-called 'transparent' sheet is made of very thin linen, and intended to work with the lantern _behind_ it, showing the picture through the linen to the audience on the other side, but this is seldom used except in the open air for religious or political meetings, &c.

An _opaque_ sheet can be had in one piece up to 9 feet square; larger sizes than this must have at least one seam, and most skilful sewing is necessary, especially with large sheets consisting of several strips sewn together.

Sheets such as these are usually supplied with either eyelet holes round the edges or else linen tapes sewn on, and the exact method of hanging must be left to circ.u.mstances. {81}

In the case of a small sheet it will be sufficient to stretch it at the four corners, and this can often be done by s.c.r.e.w.i.n.g into the walls or some convenient girder two screw eyes and similar eyes into the floor, all four being considerably farther apart than the size of the sheet.

[Ill.u.s.tration: FIG. 49.--Portable Screen Stand.]

A stout cord being then pa.s.sed through the two upper eyes, long enough for both ends to reach near the floor, one end of each can be fastened to the two top corners of the sheet and the latter drawn up, the two bottom corners being afterwards stretched and tied down tightly to the lower eyes.

In the case of large sheets this hardly suffices, and it will be found necessary to fasten the sheet at intervals all round or it will exhibit awkward creases, and this again is a matter where the lanternist must use his own initiative according to the possibilities.

In some halls the erection of a sheet in the way above described is a sheer impossibility, and in such cases a frame must be made by nailing strips of wood together, or better by utilising a portable screen stand (Fig. 49).

These stands are usually made of bamboo, with short bra.s.s connecting tubes, and the method of using them is so obvious that a description need hardly be given. The screen frames are supplied by all the leading opticians, but an intending purchaser would be well advised to see one erected before ordering. I have actually seen a 12-foot screen frame offered for sale that was too weak to carry its own weight, let alone the weight of the sheet!

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CHAPTER XI

THE PRACTICAL MANIPULATION OF A LANTERN

Having now described the optical lantern in its various forms and the more important accessories, we come to the question of practical manipulation.

In making arrangements for an exhibition the first thing to be seen to is to ensure that every accessory that will be required will be there, and the best plan is to make a complete list of all sundries to be provided. Such items as string (for the sheet), lime tongs if limelight is used, pliers for changing carbons if the arc is to be the illuminant, screw-driver, matches, the _key of the lantern box_, and other similar items, are likely to be left behind unless such a list is made and carefully checked. On arriving at the hall, the first thing to be done as a rule is to get up the sheet, after which the professional operator generally begins to feel happy again.

The next thing, if it has not been done first, is to determine the position of the lantern, and this, as has been explained in Chapter VII, is a matter of the size of picture to be shown and the focus of the objective.

It is a mistake to show too large a picture; a little 'white' round the edges is a good thing, and it is better to have a small disc well illuminated than a large one less bright. Convenience, however, must also be considered, and it is often justifiable to go back a few feet farther than other considerations would dictate in order to place the lantern in a gallery or other spot where it is out of the way.

Having fixed the position of the lantern, it should be got into place, the cable or tubing connected or whatever else is necessary, according to the illuminant to be used. It should then be lit up, the flasher of the lens opened, and the light {83} centred sufficiently to produce some sort of disc upon the screen. (It is, of course, presumed that the lenses, &c., have previously been cleaned.)

A carrier should now be placed in the stage and a slide inserted into it, and the method of doing so requires a little explanation. The slide must be placed in the carrier upside down, as will be obvious to anyone who has studied Chapter VII, but in addition to this it must be turned the correct way, otherwise the picture will be reversed from left to right. This in the case of certain subjects, such as a copy of a picture, may not greatly matter; but in slides depicting buildings or landscapes with which the audience may be familiar, or worse still, printing or writing, is a serious blunder.

Slides made by a commercial firm will usually be 'spotted,' that is to say, will have two white spots on the face of the slide when the latter is viewed in its correct position, and at the top. The slides should be turned upside down and placed in the carrier with the spots, of course, now at the bottom and _towards the condenser_.

If a slide is not spotted it should be viewed as it is to appear on the screen, and then placed in the carrier with the face that was towards the operator as he viewed it turned to the condenser, and of course inverted.

The above remarks apply only in cases where the image is thrown _on_ the screen; in the comparatively rare instances where it is shown _through_ the latter the slides must be turned round laterally, but of course still inverted. The slide having been placed in the stage it should be 'focussed'

by racking the objective in or out, and if necessary pulling out the draw tube as well until the image on the screen is sharply defined. So far the light has only been roughly centred, sufficiently so to enable the slide to be focussed, and to complete the operation both slide and carrier should next be taken out of the lantern, leaving a clear disc on the screen, and this disc may resemble any of the appearances shown in Fig. 50. {84}

If it resembles A the light must be moved to the left, if like B to the right, like C it must be lowered, like D it must be raised, always moving it to the side opposite to the dark shade until this is central on the disc. If it now resembles E, the light must be moved nearer the condenser; if, on the contrary, the centre is dark, it must be drawn back until finally the circle should be as nearly as possible clear and bright all over, as at F.

[Ill.u.s.tration: FIG. 50.--Adjustment of the Light.]