Page:Optics.djvu/142

 The effect of the camera obscura may be produced without a lens, merely by admitting light through a small aperture in a shutter into a darkened room, for the rays from each point of an external object being allowed to illuminate only a very small surface, will produce a tolerably distinct image without being made to converge, provided this image be received on a white surface, placed near the hole in the shutter. (See Fig. 167.)

157. Here a convex lens produces a distant, and therefore magnified image of a near transparent object, which is strongly enlightened by a lamp placed behind it; a concave mirror is generally put behind the lamp to throw as much of the light as possible on the object, which is some group of figures painted on a glass slide, and is inverted so that the image may be cast in an erect position, on a wall or a white sheet, at a proper distance.

In making this instrument it is to be observed, that the place of the slide must be farther from the lens than its principal focus, else the image will be thrown to an infinite distance, or become imaginary.

158. This is merely a magic lantern, in which the figures on the slide instead of being painted on the glass, are left transparent, or slightly tinted, all the rest of the glass being darkened. There is also a contrivance by which the distance of the lens from the slide is altered, when the place of the machine is changed, so as to keep the image on the fixed screen, which in this case is placed between the lantern and the spectator, and made in some degree transparent. The variation in the distance of the image from the lens, and therefore in its magnitude, is meant to give it the appearance of advancing and retiring. The deception in this, however, is incomplete, unless the brightness of the image be made to increase, instead of diminishing, as it increases in size: this may be effected by modifying the quantity of light thrown on the slide.