286 APPENDIX. 



tube drawn out each division of the eye-piece micro- 

 meter was worth T sVu inch, any object which when viewed 

 by the same objective and length of tube took up three 

 divisions of the eye-piece micrometer would measure 

 i5 S vffths or gjtfth of an inch. 



The advantage of the ej^e-piece micrometer is that 

 when its values are once ascertained the size of an object 

 can be read off at once. 



Fig. 33. 



Ocular micrometer, natural size. 



Part of the side is represented as broken away to show the field-glass at the 

 bottom, and the micrometer-glass, rn, a little below the middle The collar, 

 r serveH to vary the distance of the eye-glass from the microniPter. 



Determination of the magnifying power of a 

 microscope. The magnifying power of a microscope 

 is determined by comparing the distance between the 

 lines of the stage micrometer, as they appear imaged 

 upon the paper, when this is exactly ten inches 1 from the 

 eye, with the known interval between them. For instance, 

 if, with the high power objective and the ordinary ocular, 

 the interval of ^ 1 of an inch of the micrometer was 



1 The ordinary distance of distinct vision. 



