extremely hot, especially the back component, and hence the glass must be mounted loose in its cell, otherwise there is great danger of it cracking. Also the space between the components should be well ventilated, in order to provide for the escape of moisture which usually at the start of a lantern exhibition is deposited upon the glass, and should be got rid of before the actual lecture commences.
Even with all care, the back component of a condenser will sometimes crack, though such an accident should be a rare occurrence; and hence a professional operator will usually provide himself with a spare lens, and the condenser should be so constructed that it can readily be changed, and with as little delay as possible.
Condenser lenses as made in this country are usually ground from the glass known as 'English Crown,' and comparatively rarely crack; but they are very slightly green in colour. French condensers, on the other hand, are whiter, but the glass is more brittle, and a fracture a more common occurrence. The French variety are (or were before the war) cheaper and generally met with in cheaper instruments. More expensive lanterns are usually fitted with English condensers, as the tinge of green is almost imperceptible, and the advantage as regards greater security pretty considerable.
The Slide Carrier and Slide Stage.—Taking still the optical system of the lantern in order from back to front, we now come to the slide, slide carrier, and slide stage. The slide itself has already been described, and the carrier is simply a mechanical contrivance, usually of wood, designed for the purpose of readily changing the pictures and which in its turn fits into the stage of the lantern. It may be asked why, if slides are now always made to a standard size, the slide carrier should not itself be built into the lantern and form the stage; but the answer is, in the first place, that slides of a different size, i.e. American or Continental, may be met with,
and also that there are various mechanical slides on the market—for example, chromotropes or scientific models, such for instance as are made to illustrate the movements of the planetary bodies—and these slides are permanently mounted in wooden frames which could not be put into a carrier. The commonest form of carrier is that known as the 'Double Sliding' pattern (Fig. 37), which consists of a frame with two apertures for the slide, and an outer frame through which this itself slides and which fits the stage of the lantern.
This carrier, as will be seen, allows the next picture to be placed in position in the second aperture while the former one is being projected, and at a signal from the lecturer, the inner frame slides smoothly through the outer, and the slides are thereby changed. This carrier is simple, cheap, and quiet in its action; its one disadvantage is that each alternate slide has to be inserted from opposite sides of the lantern, and unless the operator is fairly tall this almost necessitates an assistant. Nevertheless, the carrier is the most popular of any, its other advantages, especially as regards price, being so great. It is usually constructed in such a way that the slide, as it moves out from the central position, automatically rises in its groove in order to facilitate quick removal.
Another pattern deservedly popular is that known as
'Beard's Dissolving Carrier' and is shown in Fig. 38. In this ingenious carrier all the slides are inserted from the same side, the oncoming slide being pushed in front of its predecessor, and being therefore somewhat out of focus it produces a blur on the screen.
The movement is performed by pushing in a projecting handle, and on withdrawing this the slide which is finished with comes with it, and the finish of the movement presses the new slide back until it is in its proper position and in focus.