THE EYEPIECE MICROMETER 63 



4. Repeat this procedure for each of the possible 

 combinations of oculars and objectives fitted to the 

 microscope supplied, and carefully preserve the scales 

 thus obtained. 



To measure an object by this method simply project 

 the image on to the scale corresponding to the par- 

 ticular optical combination in use at the moment. 

 Read off the number of divisions it occupies and ex- 

 press them as micro,. 



In place of preserving a scale for each optical com- 

 bination, the object to be measured and the microm- 

 eter scale may be projected and sketched, in turn, on 

 the same piece of paper, taking particular care that 

 the centre of the eyepiece is 25 cm. from the paper on 

 which the divisions are drawn. 



FIG. 56. Eyepiece micro- 

 meter, ordinary. 



FIG. 57. Eyepiece micrometer, net. 



(b) By means of the Eyepiece Micrometer. 



The eyepiece micrometer is a circular glass disc 

 having engraved on it a scale divided to tenths of a 

 millimetre (o.i mm.) (Fig. 56), or the entire surface 

 ruled in o.i mm. squares (the net micrometer) (Fig. 57). 

 It can be fitted inside the mount of any ocular just 

 above the aperture of the diaphragm and must be 

 adjusted exactly in the focus of the eye lens. 



Some makers mount the glass disc together with a 

 circular cover-glass in such a way that when placed in 

 position in any Huyghenian eyepiece of their own 

 manufacture, the scale is exactly in focus for normal 



