ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



523 



which point A B is their common tangent. If the large circle and tan- 

 gent revolve around the centre M, and if any force B C at B perpen- 

 dicular to the radius be resolved into components B D and B E, respec- 

 tively along and perpendicular to the tangent, then the component B D 

 glides along the small circle without action, and the only effective part 

 of the force is the component BE. If the radius M A be replaced by a 

 revolving arm, the tangent B A by a knife-edge, and the small edge by 

 the article to be cut, then the principle of the machine will be understood 

 (fig. 142). The excellent results obtained seem to show that this is the 

 right view of the mechanical problem. 



A horizontal beam-like arm moves on a horizontal base plate fastened 

 to the table, and can be rotated, through more than 180°, about a vertical 

 axis between two steel points, of which the lower is adjustable. The 



Fig. 142. 



arm] in its extreme part is vertically slit, and on to it the knife, with 

 Walb's fork, can be secured by means of a fly-nut working in this slit. 

 When the screw is tightened, the knife can be rotated round the axis of 

 the screw. The object-holder consists of two brass collars telescoping 

 in one another, the outer of which can be pushed into a slit of the base 

 plate, and in every position firmly screwed thereto from below. The 

 inner collar is by means of a micrometer screw rotated by baud, and 

 rises about 5 /x for every tooth of the wheel. In this inner neck is 

 the object clamp, constructed out of two wooden brass-tipped cylinder- 

 segments pressed against one another by a screw ; the edges of these 

 segments are always parallel. A spring with vertical catch, set in action 

 or released by a rod in a simple way, serves as a check to the micro- 

 meter toothed wheel. Ordinarily it lies behind the micrometer screw 

 (fig. 141), but can, by rotation of its bearer, be brought in front (fig. 142) 



