MICROMETRY — MICROMETRIC MICROSCOPES 181 



camera has been placed. Adjust the illumination even more care- 

 fully than in ordinary drawing, using axial light. Focus sharply, 

 and carefully sketch the outline of the object upon drawing 

 board or notebook, using a very hard and sharp-pointed pencil. 

 The object is now removed and replaced by a stage micrometer, 

 the instrument focused and the graduations of the scale traced 

 upon the paper, either across the outline of the object or near 

 by. The distance from the camera to the paper must be identical 

 in each case. The dimensions of the object may thus be ascer- 

 tained easily by comparison. 



The method of indicating the size of different objects in draw- 

 ings of microscopical subjects by means of tracings of a stage 

 micrometer is always preferable to a tabulation of numerical 

 dimensions, since the indication is a graphic one and appeals to 

 the eye at once. Moreover, it enables another investigator to 

 ascertain any dimensions indicated in the drawings. 



Method 3. - - Measurements obtained by means of oculars con- 

 taining ruled scales. Oculars of this type are called Micrometer 

 Oculars. There are many forms, but all fall into one of three 

 groups: (a) those having a fixed scale; (b) those in which the 

 entire scale is movable; (c) oculars having movable scales 

 actuated by micrometer screws provided with graduated heads 

 indicating the magnitude of displacement in fractions of the 

 scale divisions. 



Group b possess few advantages over Group a. Micrometer 

 oculars of Group c are generally called Filar Micrometers and 

 comprise the most accurate as well as the most convenient 

 microscopic measuring devices now in use. 



Since in micrometer oculars the graduated scale is so placed 

 as to fall in the same plane as that of the real image formed by 

 the microscope, the number of scale graduations covered by the 

 image gives a value for the size of the image only and not for the 

 object. It is therefore necessary in all cases * to first ascertain 

 the true value of the eyepiece scale with respect to each 



1 An exception to this statement is to be found in ocular micrometers with scales 

 so ruled by the manufacturer as to yield a definite value with objectives supplied 

 for use with them. 



