ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



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on raised tracks of the frame, and is not directly connected witli the screw, 

 but is simply pushed by the nut N. This arrangement makes it impossible 

 that any slight eccentricity of the screw should cause a jolting of the 

 carriage. 



The head of the screw is a solid wheel M at the end of the frame, 

 and has 250 ratchet-teeth on its circumference. The screw has 20 threads 



to the inch. The knife, therefore, is moved an inch by 20 revolutions of 

 the screw ; and as there are 250 teeth to the revolution, each tooth represents 

 1/20 X 250 = 1/5000 inch. 



The handle turns the axis J, to which is attached the wheel E. This 

 wheel is four inches in diameter, and to it is fastened the clamp which 

 holds the object to be cut. The axis also carries a fly-wheel and an 

 adjustable eccentric wheel W (figured separately). This eccentric moves 

 a lever L, the long arm of which is connected with the small chain D. 

 The chain lifts a small lever H, which works by means of a catch I on the 

 teeth of the screw-head, causing the screw to revolve. The small lever is 

 steadied and pulled back to its place by a spiral spring P, while another 

 spring catch underneath the frame prevents the ratchet-wheel from turning 

 back. By properly adjusting the eccentric wheel the levers may be made 



