38 THE MICROSCOPIST. 



a parabolic surface attached to the objective. For 

 powers, a lateral aperture above the objective has been 

 made to throw the light down through the object-glass 

 itself by means of a small reflector, as devised by Prof. 

 Smith, or a disk of thin glass, as in Beck's vertical illumi- 

 nator. This latter is attached to an adapter interposed 

 between the objective and the body of the microscope. 



Instruments for Measuring and Drawing Objects. Screw 

 micrometers are sometimes used with the microscope, as 

 with the telescope, for the measurement of objects ; but 

 the less expensive and simpler glass micrometers have 

 generally superseded them. The latter are of two sorts, 

 the stage and the ocular micrometer. The stage micrometer 

 is simply a glass slide, containing fine subdivisions of the 

 inch, line, etc., engraved by means of a diamond point. 

 Jn case the rulings are T Ji)ths and f ^ths of an inch, it 

 is evident that an object may be measured by comparison 

 with the divisions ; yet, in practice, it is found incon- 

 venient to use an object with the stage micrometer in this 

 way, and it will be found better to combine its use with 

 that of the drawing apparatus, as hereafter described. 

 The ocular, or eye-piece micrometer, is a ruled slip of glass 

 in the eye-piece. Its value is a relative one, depending on 

 the power of the objective and the length of the micro- 

 scope tube. By comparing the divisions with those of the 

 stage micrometer their value can be readily ascertained. 

 Thus, if five spaces of the eye-piece micrometer cover one 

 space of the stage micrometer, measuring T0 1 (JIJ th of an 

 inch, their value will be ^J^th of an inch each. 



Different standards of measurement are used in different 

 countries. English and American microscopists use the 

 inch. In France, and generally in Germany, the Paris 

 line or the millimetre is used. The millimetre is 0.4433 of 

 a Paris line and 0.4724 of an English line ( ,! 2 th of an 

 inch). 



In the French system the fundamental unit is the metre, 



