Optical Projection. Part 1: The Projection of Lantern Slides

by Lewis Wright; Russell Stuart Wright

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

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.

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.