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M OPTICAL PROJECTION 7. Parts of a lantern.—The diagram (fig. 8) of one of the simplest forms of lantern, once general, but now generally confined to mere toy instruments conveniently represents the es- sential parts, which we will con- sider singly. These are (1) the light L, which may or may not be supplemented by a reflecting mirror M. This, as it is the original source from which all illumination is derived, while the word * light' may need to be used in other senses, it will Fia * 8 be convenient to distinguish as the radiant ; and as it forms no part of the lantern itself, and various kinds of radiants are often employed in turn in the same lantern (as when an experiment is worked out in a small way with a lamp, and afterwards publicly performed by the oxy- hydrogen light), we will postpone it for treatment separately, and pass on here to other details. These are (2) the lantern- body B, with its chimney or heat-vent. (3) The condenser c. (4) The stage for slides or diagrams, s. (5) The focussing lens, objective, or power, p. All improvements in lanterns relate to one or other of these parts. 8. The Body.—This has two purposes : 1, to support and keep in due relation the other parts ; and 2, to prevent any light not utilised in the projection from scattering about the room and impairing the effect. Japanned tin or sheet iron is the simplest and cheapest material, and when economy is an object, will really perform as well as anything else, provided the optical parts of the apparatus are equally good. Thus, a tin bi-unial will do all that the most expensive body can ; or an experimental lantern made in this cheap way for a science school, will come short in no part of the demonstration, and may be within reach when a more expensive one would not