. Discovery. Science. DISCOVERY 173 photographic or other illustrations. The ordinary magic lantern used at lectures and other demonstrations requires darkness and specially prepared lantern slides, but the " ; as the inventor has elected to call. his apparatus, is operated in the full light of day, and is so constructed that any object of a size that can be introduced into the apparatus can be projected on to the screen or anpvhere in the room where the demon- stration is taking place. This enables the members of the audience to take notes and saves the great cost of making
. Discovery. Science. DISCOVERY 173 photographic or other illustrations. The ordinary magic lantern used at lectures and other demonstrations requires darkness and specially prepared lantern slides, but the " ; as the inventor has elected to call. his apparatus, is operated in the full light of day, and is so constructed that any object of a size that can be introduced into the apparatus can be projected on to the screen or anpvhere in the room where the demon- stration is taking place. This enables the members of the audience to take notes and saves the great cost of making lantern slides. Before giving a detailed description of the photo- graphs which illustrate this article and the various uses to which the " dussaud " can be put, let me state the principles upon which the lantern is based, in accordance with the official communication made to the Academy of Sciences. The maximum of luminous efficiency has been obtained by means of condensers with one or several lenses, the focus of which has been calculated by M. Dussaud in relation with that of the reflectors at the back of the source of light, and in such a manner as to make it as short as possible—in certain cases not more than 5 centimetres. In this way the whole of the light is concentrated by reflection on the object to be lit up and projected. M. Dussaud claims—and the demonstration before Parisian scientists proves his contention—that his optical systems realise the con- ditions of precision and yield of microscopes, and constitute, in fact, collective microscopes. With half- watt electric lamps, only 3 amperes from a voltage of no volts (the minimum quantity of voltage supplied by Parisian electric-light companies) suffice to project objects with their form, colour, relief, and movements, the dimensions of the projection being as large, if necessary, as g square yards either on a screen or on any-vertical, horizontal, or oblique surface. The apparatus is very easy to us
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