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The observer will adjust the eye glass E E, at such a distance from o o as will enable him to see the image most dis- tinctly, and the impression produced will be, that the image he looks at, is at that distance from his eye at which he would see such an object most distinctly, without the interposition of any magnifying lens; let this distance be that of a similar image oo, and the impression will be that the object he beholds has the magnitude o o. The distance of most distinct vision with the naked eye, and the distance from the image at which the eye glass must be placed to produce distinct vision, both vary for different eyes, but they vary almost exactly in the same proportion, so that the magnifying effect of the eye glass upon the image o o will be the same, whether the observer be long-sighted or short-sighted ; in estimating the mag- nifying power, therefore, of such a combination, we may consider, in all cases, the distance of the eye glass E E from the image o o to be equal to its focal length, and the distance of o o from the eye glass to be 10 inches. To estimate the entire amplifying effect of such a microscope, WP have only to multiply the magnifying power of the object glass by that of the eyeglass; thus, for example, if the distance of the image o o from the object glass be 10 times as great as the dis- tance of the object from it, the linear dimensions of the image o o will be ten times greater than those of the object ; and if the focal length of the eye glass be an inch, the distance of most distinct vision being 10 inches, the linear dimensions of o o will be 20 times those of o o, and therefore 200 times those of the object ; the linear magnifying power would in that case be 200, and, consequently, the superficial magnifying power 40000 The eye and object glasses are usually mounted at the distance of i o or 12 inches asunder, adjustments nevertheless being pro- vided, by which their mutual distance can be varied within certain limits. 470 Field glass. A convex lens is generally interposed be- tween the object glass and eye glass, which, receiving the rays diverging from the former, before they form an image, has the effect of contracting the dimensions of the image, and at the same time increasing its brightness. prev next |