234 



PRACTICAL MICROSCOPY. 



turn of the milled head, so that the number of revolutions 

 may be read off without taking the eye from the object. 



FIG. 204. 



Quekett, in his 'Practical Treatise on the Use of the 

 Microscope,' has shown the theoretical possibility of measur- 

 ing a space so small as the one eight hundred thousandth 



of an inch with the 

 eighth objective and 

 this micrometer ; but 

 there are many practi- 

 cal difficulties in the 

 way of attaining to this. 

 MEASUREMENT OF 

 ANGULAR APER- 

 TURE. Under the 

 head of microscopic 

 FIG. 205. measurements may be 



described the method of using Prof. Abbe's apertometer 

 for measuring the aperture of objectives, a rough method 

 of accomplishing this having been already described on 

 page 56. An illustration of this instrument may be seen in 

 Fig. 206. It is a semicircular disc of crown glass, 3 5 inches 

 in diameter, and half an inch in thickness, polished on the 

 cylindrical edge and ground to an angle of 45 upon the 



