46 
ACCESSORY APPARATUS. 
Fig. 18. 
Fig. 19. 
A portion of the field of view on one side is cut off at right 
angles to the cobweb threads, Fig. 19, by a scale formed of a thin 
plate of brass having notches at its edge, whose distance cor- 
responds to that of the threads of the screw, every fifth notch 
being made deeper than the rest for the sake of ready enumer- 
ation. As each notch corresponds to one turn of the screw, the 
number of turns can be read off in the field of the instrument, 
and the fraction of a turn on the graduated head. 
By this simple contrivance, the distance of the threads can be 
J. & W. GRUNOW & GO’S ILLUSTRATED 
