158 THE HISTORY AND DEVELOPMENT OF THE MICROSCOPE 



There are, however, two kinds of tube-length : (1) an optical and 

 (2) a mechanical. 



The optical tube-length is measured from the posterior principal 

 point of the objective to the anterior principal point of the eye-piece. 



Hie mechanical tube-length should be measured from the top of 

 the tube into which the eye-piece fits, and upon which the bearings 



FIG. 123. Old Ross stand (1842), rotating foot below the pillar. From the cabinet 

 of the Royal Microscopical Society. 



of the eye-piece rest to the end of the nose-piece into which the 

 objective is screwed. 



Unfortunately different makers estimate tube-length differently 

 and take different points from which to make their measurements. 

 Looking at the matter broadly, there are two estimates for tube- 

 length in practical use : these are the English and the Continental. 



