The book of lantern ; being a practical guide to the working of the optical (1888)

THE BOOK OF THE LANTERN. 25 effect whatever upon the performance of the lens, whether used with a camera, or as a lantern objective, supposing it to be suitable to lantern work in other respects. But transfer the bubble from the objective to the condenser, and it at once constitutes a real eyesore, which will be terribly magnified on the screen. For this reason lantern owners should take the greatest care to prevent their condensers becoming scratched, for a mark hardly visible on the glass will become fatally apparent on the screen. It will be seen, from what has gone before, that the duty of the condensing lens is to take up and utilise as large a bundle of light rays as is practicable, and with those rays to brightly illuminate the whole of the picture or slide placed against it. The other part of the optical system of the lantern is the objective lens, which is destined to form a magnified image of that picture or slide. To this important part of the apparatus I must now turn the reader's attention. As the duty of the condenser is to give the greatest amount of illumination to the lantern picture or slide, so the province of the objective lens is to form as perfect as possible a magnified image of that picture upon the screen or sheet placed for its reception. Toy lanterns are fre- quently fitted with a double convex, the worst form of all, or with a plain convex lens, which is little better. With regard to lanterns of more pretension, we find that different makers adopt different forms and combinations for their objectives. Some use a couple of plano-convex achromatic lenses, in conjunction with a stop or diaphragm, the flat sides of the lenses being next the light; a very