ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



363 



prefers to stand near the screen, to have control over the fine adjustment 

 of the ocular so that he may arrange a sharp definition of any desiiv.l 

 part of the image. It is in actual operation at the Botanical Laboratory, 

 Groningen. Dr. Moll has designed a sort of wheel-and-axlo arrange- 

 ment (fig. 78) consisting of two grooved discs of radii 2*1 and 10*5 cm., 

 rotatory about a horizontal axis and firmly fastened together. They are 

 enclosed in a metal frame which is secured to a convenient beam in the 

 ceiling. The cord b is fastened at one end to the small disc, and, at 

 the other, to the ocular mount. To the larger disc is fastened the 

 vertical cord terminating in a handle c, whicli is gripped by the lecturer. 

 The size of this larger disc is important as it increases the delicacy of 



Fig. 79. 



the ocular fine adjustment. The range of this latter is 5-5 cm. ; but 

 the corresponding stroke of the lecturer's handle is about 26 cm., and 

 the precision attainable in image definition is proportionately increased. 

 In many cases, and especially at the beginning of a demonstration, a 

 medium position would be convenient : this is attained by making the 

 handle terminate in a ring d, which may be then secured to a nail e i in 

 the wall. For better attainment of this medium position an adjustable 

 rod / is contrived in the hollow handle, and its suitable length deter- 

 mined by a screw g. ... 



To the ocular mount itself (fig. 79) strong spiral springs s are 

 attached. These work in a frame at whose extremity are a pair of 



