Optical Projection Part 9

It is important to note that this adjustment _cannot_ be properly made while a slide is in position, and neither can it be made until the lantern has been focussed, so the above procedure is the only way to get a satisfactory result. With some of the larger illuminants, such as a paraffin-oil lamp, there are no centering adjustments, the size of the radiant rendering exact centering unnecessary, and generally speaking the smaller the luminous point, the more exact must the operation be.

In the case of such illuminants as acetylene or limelight {85} care must be taken that they are turned fully on before centering, otherwise turning on the fuller amount afterwards will raise the position of the luminous spot.

The centering achieved, the slide carrier may be replaced, the first slides placed in position, the remainder arranged in their proper order, the system of signalling with the lecturer determined, and all is ready.

If there is still an interval before commencing, the light may be switched off or turned out, or in the case say of limelight, turned down very low until wanted.

It is of extreme importance to see that all the slides are in their right order, though the duty of seeing to this usually rests with the lecturer rather than with the operator. I remember hearing of one lecture on the life of Queen Victoria, when the lecturer announced, 'The next picture will be a photograph of the Royal Prince who for many years shared the Throne with our gracious Sovereign.' At the words the operator brought on the next slide, which proved to be _a restored specimen of a prehistoric monster_ (tableaux!). Such mistakes 'bring down the house,' but in serious lectures, and especially at religious services, cannot be too carefully guarded against.

Mention has already been made of the liability of moisture to condense on the surfaces of the condensers or slides, and to avoid this, so far as the condensers are concerned, it is well to light up say ten minutes before the lantern is actually wanted, or alternatively to take out the condensers and thoroughly warm them in front of a stove, or to place them wrapped in a cloth on hot-water pipes. The slides should in the same manner be warmed before using and should be finally held above the lantern or placed on the top, if this is flat, the last thing before being placed in the carrier. If these precautions are omitted, on a cold night the first surface of the condenser will become so covered with moisture as to almost obscure the slide, and this will quickly disappear {86} with the heat of the lantern.

Next, the two inner surfaces of the condensers will behave in turn in the same way, and will take considerably longer to clear, especially if the ventilation of the condenser is poor; then the fourth surface will take up the running, and finally, when the lanternist is congratulating himself that the trouble is over, each successive slide will become affected in the same way. With an operator who knows his business, none of these troubles should occur.

ACCIDENTS.--These will occur sometimes, even in the best managed exhibition; the rubber tubing feeding a limelight jet gets kinked or trodden on, or a fuse melts if electric light is being used, &c., and out goes the light. In such cases a loud request such as, 'Would you mind turning up the light for a minute, please,' accompanied by a good-humoured laugh, usually allays the fears of 'nervy' people. An operator must never get 'nervy' himself. I have known of more than one fias...o...b..cause some little hitch occurred, and two or three timid ladies crowded round and asked anxious questions, till the lanternist lost his head. In one such case the cautious superintendent at a children's entertainment decided that it would be safer not to have the exhibition at all, simply because a regulator was not screwed tightly enough into a cylinder to prevent an escape of gas, only the operator (a somewhat youthful one) had been driven to the verge of lunacy by continual questions of the standard type, 'Are you sure it is safe?' 'Will it blow up?' 'Are you certain you understand it?' &c., &c. More serious accidents, such as the entire lantern getting upset, ought never to occur, and it is up to the lanternist to take whatever precautions he deems necessary to safeguard his instrument. With a juvenile audience, for example, it is often a good thing to arrange a barricade of forms round the lantern and to see that no one comes within it.

Finally, 'whatever is worth doing at all is worth doing well,' and this is as true of lantern exhibiting as of anything {87} else. There are a deplorable number of lantern exhibitions given with the sheet hanging in creases, dirty lenses, light poorly adjusted and centred, and occasionally slides shown upside down. A conscientious lanternist should see to _every_ detail; slipshod methods, as in everything else, mean poor results.