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And magnified images

Projection Lantern. - The projection lantern, which is used to throw an image of a brilliantly illuminated object on a screen consists of a powerful source of light S (Figure 41), a large condensing lens NP, and a front or objective, lens LM. The condensing lens NP collects the light from the source S and sends it through the slide OI, so that this slide is brilliantly illuminated. The objective LM then produces a real image of the slide OI on the screen XY. Since the slide OI is just outside the principal focus of the lens LM, the image on the screen is enlarged. This image is also real and inverted. The magnification produced by the lantern is obtained from an application of the law for the magnification of a lens.

Simple Microscope. - When an object is placed slightly nearer to a converging lens than its focus, an eye brought up to the lens sees a virtual, erect, and magnified image A'B' (Figure 42). A lens used in this way constitutes a simple microscope. In order to obtain the great­est advantage, the eye should be as near as possible to the lens. In this way, the field of view is made as large as possible, and the distance of the virtual image A'B' from the eye for distinct vision is made as large as possible.

In order to find the magnification of a lens used in this way, it is necessary to consider that the apparent size of an object is determined by the angle it subtends at the eye. Now for most distinct vision, an object must be about 10 in. from a normal eye. If an object is placed at a greater distance than this, the image on the retina is smaller and its details are not seen so distinctly. When the object is placed nearer than 10 in., the image on the retina is blurred. When an object is examined with the aid of the magnifying glass, the object is brought nearer to the eye than would be possible for distinct vision without the magnifying glass. In Fig. 42 the angles subtended at the center of the lens by the object AB and the image A'B' are the same. But OD is the distance of distinct vision, and, if the lens were absent, the eye could not see AB distinctly until it was removed to ab.

A'

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