A companion to the United States pharmacopia; . Pig. 551. -Action of Lenses in Compound Micro-scope, simplest form. standard length of tube obtained in that manner. This, however, isonly necessary when we desire to make measurements. By drawing-out the tube we may increase the rate of enlargement. By using oculars of different magnifying powers we can obtain va-rious degrees of enlargement (or powers) with the same objective, butit generally is better to obtain higher powers with higher objectives. 1040 A COMPANION TO THE The power of an instrument, or rather of the combination of lenses atany
A companion to the United States pharmacopia; . Pig. 551. -Action of Lenses in Compound Micro-scope, simplest form. standard length of tube obtained in that manner. This, however, isonly necessary when we desire to make measurements. By drawing-out the tube we may increase the rate of enlargement. By using oculars of different magnifying powers we can obtain va-rious degrees of enlargement (or powers) with the same objective, butit generally is better to obtain higher powers with higher objectives. 1040 A COMPANION TO THE The power of an instrument, or rather of the combination of lenses atany time attached to the microscope, is expressed in diameters and notin areas. Thus, if an object which in reality is one-tenth of an inchsquare, appears to be five inches square when viewed with a microscope,it is enlarged fifty diameters, commonly expressed fifty times, but as. ->/ Fig. 552.—Action of Field-lens in Compound Microscope. it is equally enlarged in all directions its area will appear to be 50 X 50,or 2,500 times as large as it really is. Dealers often state the surfaceenlargement when advertising cheap popular instruments, because amicroscope magnifying 2,500 times will sell more readily for a fewdollars than another which magnifies only fifty diameters. In reality the rate of enlargement is still greater than expressed UNITED STATES PHARMACOPOEIA. 1041 above. A small object, as a pollen-grain, viewed with a magnifyingpower of fifty diameters appears enlarged equally in all of its dimen-sions, and therefore seems to be 50 x 50 x 50, or 125,000 times as largeas it really is. It is well to remember that an excess of light is not always the bestway of showing delicate structures, but that a dim light will oftenbring out details, totally invisible in a full flood of central light. Thediaphragm under the stage enables us to moderate the light as
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