Tiie Microscope. 223 



at the end of the shaft above described, moves between two 

 stops in a solid, upright plane, the distance between these stops, 

 and hence the movement of the lever, can be regulated with 

 great precision, by means of a screw with a finely graduated 

 head. By this means, the extent of motion of the precision 

 screw can be regulated with the greatest exactness. The number 

 of movements of this lever, and hence of the lines ruled, is regis- 

 tered by an index having on a face like a watch and situated over 

 the long arm of the lever. 



Underneath the index wheel of the precision screw and in 

 accurate contact with the edge of it, are two other electro-mag- 

 nets, which, when their circuit is closed, hold the index firmly 

 in its then position. Over the index-wheel of the precision 

 screw is a. microscope, with an eye-piece micrometer of 100 equal 

 spaces, so adjusted that the end lines exactly coincide with the 

 lines bounding one of the 100 equal spaces into which the 

 index wheel is divided. The pitch of the screw being 1-20 inch, 

 and the index wheel being divided into 100 equal parts, one of 

 the divisions of the eye-piece micrometer obviously represents 

 a movement of the table bearing the plate to be ruled of 

 1-200,000 of an inch. 



Dismissing for the present the important question of cor- 

 rections for errors of the screw, suppose that a plate is in position 

 upon the table, that the several indexes are at zero, and that it 

 is desired to rule any number of lines 1-100 mm. apart. The 

 value of each division of the index, regulating the distance be- 

 tween the stops which limit the movement of the long arm of 

 the lever, having been previously ascertained by comparison and 

 computation, this index is set at the proper figure and the machine 

 is started. At the proper part of the revolution of the shaft, 

 the circuit connected with the electro-magnets at the end of the 

 short arm of the lever, is closed (the circuit connected with the 

 electro-magnets under the index-wheel being broken) and the 

 short arm of the lever is firmly fastened to the edge of the index 

 wheel, the cam, revolving at the end of the shaft moves the lever, 

 and through it the index wheel of the screw, through a distance 

 representing a motion of the screw of 1 -100 mm. At this point the 

 first circuit is broken and the second connected with the electro- 

 magnets and the index wheel is closed, and the wheel 



