The entire lens therefore mounted in its tube resembles a tunnel of varying length according to its focus, and through this tunnel a cone of light rays have to be passed. It is plain, therefore, that a lens of long focus, which in practice means a long tube length, must be made also of large diameter, or a portion of the cone will be cut off, with a consequent loss of light.
In practice lenses up to 6 inches focus are usually made of 2 inches diameter, and there is no advantage in a larger size. With a lens of 8 inches focus there is a slight gain in increasing the diameter to 2⅜ (the next 'standard' size), and lenses of longer focus than this should certainly be 2⅜ inches up to, say, 12 inches focus, when a lens of 3 inches diameter is preferable. These large lenses are, however, very expensive, both in themselves and also on account of the fact that their weight entails heavy and expensive brass mounting, and hence lenses up to 14 or 15 inches focus are often supplied in the 2⅜ size for reasons of economy.
To sum up, short-range lanterns, as they are called, are usually fitted with lenses of 2 inches diameter, and long-range instruments either with 3-inch lenses or the intermediate size of 2⅜ inches. If a lantern is purchased for either long or short-range work, it is usually fitted with a brass front for a large lens, and so arranged that a shorter focus lens of 2 inches diameter can easily be interchanged, utilising the same brass mounting.
Lenses of variable focus have also been designed, in which an additional lens can be added or subtracted to increase or decrease the focal length; but nothing very practical has yet been achieved in this direction, and therefore these 'Omnifocal' lenses have never come into general favour.
Objectives like condensers want cleaning at times, and care must be taken not to scratch the glass, as the concave lens of each component is of flint glass, and very soft. A
clean chamois leather is the best thing to use, but a soft cloth, or even a handkerchief, may be employed with care. It is very important that a lens be reassembled, after cleaning, the correct way, as a single lens reversed would utterly spoil the definition. The front component is usually balsamed together, and therefore all that is needed is to see that the whole combination is not reversed. In the Petzval system this lens should have its convex constituent towards the screen (Fig. 41). The back combination is usually loose, and the two lenses are sometimes separated by a thin brass ring. In the Petzval lens the concave element should be inside, with its concave surface outwards, the deep curve of the other lens should fit into this concavity, and the flatter curve face towards the condenser. One or two makers, however, have introduced a modification of the Petzval system in which the whole of this back combination is reversed, and the exact arrangement should therefore be noted very carefully when taking the lens to pieces.
CHAPTER VIII
THE BODY OF THE LANTERN
We now come to the mechanical construction of the optical lantern, and a great variety of design presents itself, according to price, type (i.e. short range or long range), and the individual ideas of the various makers.