ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



479 



carrier, and accordingly it became possible to construct a more delicate 

 movement. 



Fig. 75 is a vertical section through the mechanism of the new fine- 

 adjustment. The essentially new element of the movement is a heart- 

 shaped cam /composed of two symmetrical and equal parts of a spiral 

 curve. The spirals are of that simplest order in which the distance of 

 the periphery from the centre of rotation increases in the same ratio as 

 the angle of rotation. The maximum change in the peripheral distance 

 amounts to :^ mm. The heart-shaped piece is mounted upon a tooth- 

 wheel d in such a manner that its centre of rotation coincides with that 

 of the tooth-wheel. The tooth-wheel is actuated by an endless screw a, 

 which is controlled by the two milled heads projecting from the sides of 

 the body. A spring h exercises a constant pressure upon the screw- 

 spindle and the tooth-wheel, and thus eliminates play, even when the 

 motion is reversed. The rise and fall of the spiral is transmitted by a 



Fig. 75. 



roller g upon a carrier h, which, being rigidly attached to the sliding 

 piece, communicates its motion to the tube. The roller and spiral are 

 kept in permanent contact by the Aveight of the tube and the spring 

 situated above the roller and depressing the latter. An elevation of 

 3 mm. of the roller and consequently of the tube is produced by half a 

 rotation of the tooth-wheel, i.e. a rotation of the cam from the point to 

 the base. The tooth-wheel having 60 teeth must accordingly be turned 

 through 30 teeth. Eotation through one tooth thus gives rise to an 

 elevation of the tube amounting to 3/30 = 0*1 mm. The displace- 

 ment of one tooth is effected by one complete revolution of the worm- 

 screw a and its milled head. " The drum is mounted upon the screw 

 spindle and divided into lOO equal parts. The rotation of the drum 

 through one division corresponds accordingly to an elevation of 



^ = 0-001 mm. 



In addition to the delicacy and slowness of the movement which is 

 attainable with this screw, it possesses several other advantages. Owing 

 to the association of two symmetrical spirals, so as to form a continuous 



