288 TECHNICAL MICROSCOPY. 



IV. 

 THE MEASUREMENT OF MICROSCOPIC OBJECTS. 



WE need not expatiate on the importance of the determination of 

 the magnitudes of the objects in microscopical investigations ; nor 

 is it necessary to describe in detail the different micrometric 

 methods which have been proposed from time to time. We confine 

 ourselves to measurements with the glass micrometer and screw- 

 micrometer, and to the determination of angular magnitudes by 

 the goniometer. 



The glass micrometer, as generally employed at the present time, 

 consists of a glass plate applied within the eye-piece mounting, or 

 arranged to slide into it ; sufficiently fine divisions are ruled upon 

 it with a diamond. The larger spaces are denoted by longer 

 division-lines, as on an ordinary scale (Fig. 163) ; the divisions 

 into square meshes formerly made appear to have gone out of date. 

 The eye-piece micrometers of the German and French opticians 

 are generally constructed on the metric scale, 

 and in most cases are divided into spaces of 

 one-tenth millimetre. 



It is, of course, evident that the divisions 

 of the eye-piece micrometer can, in the first 

 place, serve for measuring the magnitude of the 

 real image only. In regard to the object itself, 

 FIG. 163. their value becomes the less the higher the 



objective amplification, and must therefore be 

 specially determined for the individual objectives. If, for instance, 

 the magnifying powers of two objectives, in combination with the 

 eye-piece, are taken respectively as 20 and 50, a length of 1 mm. 

 seen on the eye-piece micrometer corresponds to a length of 5 mic. 

 and 2 mic. in the object. 



To determine the relative value of micrometer-divisions, it is 

 best to use a stage-micrometer a small glass slip, upon which the 

 millimetre is divided into 100-250 parts or more, so that the 

 distance of the division-lines = 10 to 4 mic., or even less. If we 

 place one of these stage-micrometers under the Microscope, and 

 adjust the eye-piece micrometer so that its divisions can easily be 



