INVERTEBKATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



717 



It will be readily iinclcrstood from the foregoing description that 

 by pulling downwards upon the cord a rotary movement will be 

 imj)arted to the pulley D, and through the pawl and ratchet teeth to 

 the turntable E. By relaxing the pull upon the cord, the spring F 



Fig. 65. 



will draw the cord back again, carrying round the pulley D and pawl 

 d in the reverse direction. The weight and momentum imparted to the 

 turntable will be sufficient to maintain its rotation during this backward 

 movement of the pulley, and by a repetition of the process the rotation 

 of the turntable may be continued as long as desired. 



In order to hold the slide in a central position upon the turntable, 

 two metal slips // are pivoted at their oi)posite ends to the surface 

 of the table. These slips are provided with springs which bear against 

 the heads of screws on the upper side of the table. By this means the 

 slide will always be accurately centered laterally. Its adjustment 

 longitudinally can be effected by the aid of a ring engraved in the 

 surface of the table. 



It is often desirable, however, to apply a circle of cement excentri- 

 cally. This is especially the case in re-ringing old slides. The 

 adjustment longitudinally can be readily made as already cxphiiued, 

 and can of course be made either central or excentric. To provide 

 for an excentric adjustment laterally, tlie toj) of the table is formed of 

 a movable disk, which is jjivotcd at c\ This disk is secured in a 

 central position by mcaiis of a millcd-lieadcd screw e-. By withdraw- 

 ing this screw slightly the disk is set free and can be shifted laterally 

 as indicated by the dottetj lines in Fig. G4, so that any amoinit of 

 exceutricity can bo given to the disk, and consequently to the slide 

 which it carries. 



It is obvious that the pulley G may be placed elsewhere on the 

 base-board A, or may be disjjeused witli altogether, in wliich case a 

 direct pull upon the cord can bo used to impart motion to the turntable. 



