TUBE LENGTH 71 



of 160 mm., but others use 170 mm. or 200 mm. The 

 corrections of high-power objectives are so delicate that 

 care must always be taken to adjust the tube length to 



At 





/<r 



F 



T T 



Lower focal plane of the eyepieces 



Upper focal plane of Objective JVo. 3 



* 



Lower focal plane of the compound Microscope 

 Fig. 22 



MECHANICAL AND OPTICAL TUBE LENGTHS 



the objective in use, and to correct for any variation from 

 the standard thickness of cover-glass if the objective has 

 no correction collar. The thickness of any nose-piece or 



