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ON PROJECTION 13 illumination of the ' disc' (or lighted area) on the screen. And in microscopic projection, where definition in the image is so much more severely tested, it often becomes necessary practically to ' focus ' the light itself upon the object, in order that the rays diverging again from that focus as from a new point, and the image-forming rays from the object itself, may practically coincide. Such are the elementary principles and conditions of Optical Projection. Simple as they may seem, for want solely of a due understanding of them, many demonstrators never half develop the powers of their apparatus. This does not apply only to so-called itinerant lecturers; for I have repeatedly seen polarisation and other physical phenomena projected, with the most elaborate electric-light apparatus, at what are considered the very head-quarters and principal arenas of scientific exposition, in a manner inferior to what I had been accustomed to obtain with only oxy-hydrogen illumination. These principles are the essential key to the whole of what follows; and both excellence of apparatus, and success in using it, depend upon their being thoroughly grasped, in the first place by the optician, and in the second place by the operator who uses the apparatus which the other has con- structed. CHAPTER II THE PABTS OF A LANTEEN A LANTEEN is an optical apparatus so arranged, with all its parts approximately fixed in their places, that pictures or apparatus can be exhibited on a screen with the least and most convenient manipulation. In this place we will consider only the exhibition lantern, for the projection of slides or diagrams, leaving other apparatus for separate consideration in a chapter devoted to experimental lanterns.