. The principles of physics. The linear dimensions of an object and of its image formedby a convex lens are proportional to their respective distancesfrom the center of the lens. The image is virtual or real, erector inverted, according as it is on the same side of the lenswith the object or on the opposite side. VIRTUAL IMAGES. 365 325. Virtual images. — Since rays that emanate from apoint nearer the lens than the principal focus diverge afteregress, it is evident that their focus must be virtual and onthe same side of the lens as the object. Hence, t?ie image ofan object placed nearer the le
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. The principles of physics. The linear dimensions of an object and of its image formedby a convex lens are proportional to their respective distancesfrom the center of the lens. The image is virtual or real, erector inverted, according as it is on the same side of the lenswith the object or on the opposite side. VIRTUAL IMAGES. 365 325. Virtual images. — Since rays that emanate from apoint nearer the lens than the principal focus diverge afteregress, it is evident that their focus must be virtual and onthe same side of the lens as the object. Hence, t?ie image ofan object placed nearer the lens than the principal focus isvirtual, magnified, and erect, as shown in Fig. 275. A convexlens used in this manner is called a simple microscope. 326. Simple microscope. As its name implies, the micro-scope is an instrument for viewing minute objects. Thesimple microscope consists of a single converging lens soplaced that the object is between the principal focus and thelens. It magnifies by increasing the visual angle. A. B Fig. 275. The magnifying power of the lens is simply the ratiobetween the apparent diameter of the image and the diameterof the object, e.g. AB: AB (Fig. 275), or it is the ratiobetween the visual angles under which the eye would seeimage and object, if both were placed at the distance ofdistinct vision.^ If the lens be of short focus, as is usuallythe ease, the magnifying power is approximately the ratio ofthe distance of distinct vision to the focal length. Thus alens of in. focal length would magnify 20 to 24 diameters. • For normal eyes, an object to be^ seen most distinctly must be placed at adistance of 10 to 12 inches, hence this is regarded as the distance of distinct vision. 366 ETHER DYNAMICS. 327. Diverging lenses. — Since the effect of concave lensesis to render transmitted rays divergent, pencils of raysemitted from A and B (Fig. 276) diverge after refraction, asif they came from A and B, and the image will appear to beat A B. Hence, i