Magic Lantern is an optical instrument for throwing on to a screen magnified images of pictures which are painted or photographed on small squares of glass and are called "slides." The date of its invention is uncertain, but an indication of it is to be found in the writings of the Jesuit Kircher, who lived in the 17th century. Until about thirty years ago it was used chiefly as a plaything for showing comic pictures, or as a means of manifesting so-called magic phenomena. Owing chiefly to the advance in photography, lantern slides are now largely used by lecturers in place of diagrams, while a slight alteration in the construction of the lantern makes it possible for scientific experiments, done on a small scale, to be rendered visible to a large audience. In its simplest form the lantern consists of a box containing a source of light with a chimney above it, a reflector behind it, while in front of it is a circular opening fitted with a brass tube. At the inner end of the tube is a lens called the "condenser," then comes a slit for the reception of the slide, and beyond that another lens called the "objective," in the focus of which, between it and the light, the object is placed. The screen is placed some distance in front of the objective. An oil-lamp was at one time the only source of light used. An improvement upon this was the argand gas-burner, but the best results are obtained with limelight or electric light. The condenser causes the light to illuminate the slide to a very high degree, yvhile the objective forms an image of it on the screen. The slide must be put in the lantern upside down, as the image on the screen is inverted. The objective can be moved towards and away from the slide, and so enable one to focus the instrument and obtain a clear and distinct image. A lantern can only be used successfully when worked in a darkened room.