ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



495 



length of 50 mm. and a pitch of - 5 mm. Certainty of screw action is 

 attained by a counterweight, and the reading (0 • 0005 mm.) is given by 

 a loup or two drums with common index, as in model C ; there is also a 

 scale for reading the millimetres. The Microscope is in a slide, and is 



Fig. 125. 



adjustable perpendicularly to the direction of measurement ; it is operated 

 by a screw of 50 mm. available length and 1 mm. pitch, which can 

 therefore be used as a measuring screw. The corresponding drum is 

 divided into hundredths, and by estimation of tenths readings can be 

 taken to 0*001 mm. A laterally applied millimetre scale counts the 

 whole rotations of the screw. The Microscope is equipped with a 



