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34 LANTERN-SLIDE MAKING AND EXHIBITING. any room of ordinary dimensions, and it is by no means necessary to cover this with the picture. Most slides of any artistic pretensions are now masked with oblong openings, and if the position of the lan- tern is arranged so that the longer side of the projected image is about 4ft. Gin. or 5ft. in length the best possible effect will be obtained. The size of the picture is regulated Factors influ- by two f acto rs, the distance of the encing the Size •, £ ,-, j ,-, , «*i. ™- * lantern from the screen, and the focus ol the Picturei of the lens. The further the lantern is moved from the screen the larger will be the image. With regard to the lens the longer its focus, work- ing at any given distance from the screen, the smaller the picture, and vice versa. The lens usually sup- plied with lanterns of the ordinary type is of about 4^in. focus, which is too short, as it necessitates bringing the lantern too close to the screen to obtain a disc of moderate dimensions. Therefore, in buy- ing a lantern stipulate for a lens of about 6^in. or Tin. focus. The reader will be governed in his Choice of , . . .„ . ' , , . Illuminant. choice ot an illummant by local cir- cumstances, but if the electric cur- rent is available preference should be given to the Nernst lamp. It is an ideal illuminant for home use in the lantern and surpasses all others in conven- ience, comfort, and efficiency. Next in point of utility comes acetylene, which can be used anywhere in places where neither gas nor electricty are avail- able, but it has the disadvantage that more trouble is involved in cleaning the apparatus after use, and this, if success is to be attained, must be attended to. After this comes the incandescent gas light, far more