350 THE MICROSCOPE. 



words are necessary on the specific definition of the magnifying power 

 of a microscope. To ascertain that power, let us observe through the 

 microscope a scale minutely graduated on glass ; it is not difficult for a 

 mechanist, with the help of rulers moved forwards by means of fine 

 screws, to graduate scales on glass plates with the diamond, whose 

 single divisions shall be t l, or even ytqo* °f a ^ ne from one another. 

 Let us fix near the microscope, at the distance before the eye of distinct 

 vision, a scale proportionally divided, only more coarsely: let us say 

 into lines. Commonly ten inches, or as well twenty centimeters, are 

 taken as the distance from the eye. It may now be determined, by 

 visual measurement, how many divisions of the fine and magnified 

 scale apparently occupy the same space with the divisions of the larger 

 scale seen with the naked eye. Let, for example, the fine scale be 

 divided into hundredth parts of a line, and let two divisions of this 

 scale, viewed through the microscope, occupy the same extent as eight 

 lines on the scale seen with the naked eye: the microscope thus mag- 

 nifies 1 -§-° X 6 = 300 times. This is the linear magnifying power of 

 the microscope; and since it affords the simplest expression of the prac- 

 tical performance of the instrument, it is that which is usually specified 

 by scientific observers. It is now plain, that a square y^o of a line in 

 length and breadth, seen under such enlargement, will appear one 

 line long in each direction ; that thus on its surface, 90,000 squares, 

 each -j-J^ of a line in length and breadth would find room. The super- 

 ficial enlargement by the instrument would hence be 90,000 fold. 

 Instead of the square let us assume a cube with sides r J 7 of a line in 

 extent, and through the 300-fold linear enlargement this cube would 

 appear of such an extension that twenty-seven million cubes, of T ^o of 

 a line lateral measure, would be contained in it. The magnifying 

 power of the instrument in respect to the whole mass is therefore 

 27,000,000 fold. In this way do the perambulating microscopists 

 obtain their loud-sounding million-times-magnifying numbers. A hun- 

 dred fold linear enlargement itself gives one million fold, two hundred 

 ibid gives eight million fold, cubic measure. 



It is in the nature of things that the images of the most perfect ex- 

 isting microscopes, the compound, of which we have been before speak- 

 ing, can be seen with but one eye at a time. A microscope, to serve 

 for exhibitions, whose figures are to be seen by many persons simulta- 

 neously, must have an essentially different construction. The sun or 

 hydro-oxygen gas microscope (they differ from one another only as 

 regards the source from which the light issues) is, in essentials, simi- 

 lar in arrangement to the well known child's toy, the Laterna magica. 

 The wdiole difference lies in the greater intensity of the light employed, 

 and the careful management of the magnifying glasses. The rays of 

 the^ sun received upon a mirror, or those from a cylinder heated to 

 whiteness by the hydro-oxygen blowpipe, are thrown, after being con- 

 centrated by convex lenses, on the exhibited object: the rays pro- 

 ceeding from this now pass through a system of achromatic lenses, in 

 all points equivalent to the object-glass of a microscope of the ordinary 

 sort, composed of several lenses. Since the object is now placed some"- 

 what more removed from the object-glass than its focal distance, there 



