Such a lantern is hardly suitable for a powerful arc lamp or limelight jet, or for heavy long-range lenses, but is a very good typical instrument for use in moderate-sized halls, and a lantern of this general type is usually found in lantern catalogues, though, of course, the exact designs vary according to the ideas of the manufacturer. Of lanterns for long-range work a good example is perhaps Messrs. Newton & Co.'s 'Intermediate' pattern (Fig. 43).
This again is only typical of many others by the various makers, but the principal points are common to all. These are: (1) The large and well-ventilated body; (2) the long
baseboard; (3) the strong and massive brass front necessary to carry the large long-range lenses; (4) the velvet curtain at the back to close in any stray light from a powerful arc lamp.
The two foregoing designs are perhaps sufficiently typical of lantern bodies in general to make further detailed description of individual designs unnecessary; but reference should be made to features which special requirements may render advisable.
Under this heading mention must be made of Bi-unials or Double Lanterns, as used for the once famous 'Dissolving Views.'
A bi-unial lantern consists essentially of two different instruments, each complete with its limelight jet or other illuminant—front, condensers, objective, &c., usually mounted on one body—and with some arrangement for 'dissolving' or turning the light in each lantern gradually on and off.
Fig. 44 shows the back view of such a lantern with two limelight jets and dissolving tap, this piece of mechanism (shown below in the illustration) being so arranged that when the lever is horizontal both lanterns are on full, but moving the lever either way cuts off the gas supply to one lantern. In the case of limelight the tap should always operate by cutting off the oxygen supply in advance of the coal gas (in order to avoid a 'snap'), and the latter should never be cut off entirely, but a small bead of flame left to keep the jet alight, until the lantern is required for the next slide.