ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 1055 



Fig. 229. 



Fig. 230. 



Fig. 231. 



syringe (the Lacaze-Dutliicrs vertical injecting syringe is the most 



convenient for this purpose). By working the syringe water can bo 



forced into the tube or witlidrawn from it, 



and, as before stated, the focus of the 



Microscope is varied. A cover (fig 231) 



can be used to exclude the light from 



entering the body-tube through the slit. 



We will assume tliat by this means tlio 

 variation in tlie focus can bo made witli 

 much more sen.sitivencss than with the 

 best mechanical means, though tlie latter 

 has now reached a gr(;at pitch of perfec- 

 tion. Is this (assumed) increased sensi- 

 tiveness olftained at the sjicrifice of (^ther 

 indisiKJiisable (jualities? There can be no 

 di^ubt that it is. Tlie arrangement is of 

 course of no use except with liigli powers— f.>r low powers the existing 

 ffXiusing arrangements leave nothing to ho desired as a practical 



