ZOOLOGY AND BOTANY, MICROSCOPYj ETC. 



817 



Fig. 178. 



can be raised or depressed. With a No. 10 immersion of Hartnack, 

 for instance, a change in the image obtained with -^V or -J^ of a turn 

 of the ordinary focussing screw is accomplished only by a whole turn 

 of the milled head d. 



Mr. J.Deby* has also recently made a somewhat similar suggestion; 

 he says : " When allowing all but adepts in the use of the Microscope 

 to peep through my high-power 

 glasses, I have often felt a certain 

 degree of uneasiness, not to say 

 of alarm, regarding the fate of 

 valuable test-slides, or still more 

 valuable objectives. Many others 

 have no doubt experienced the 

 same discomfort, which I find 

 an easy matter to attenuate to a 

 considerable extent, by focuss- 

 ing from the eye-piece instead 

 of from the coarse or the slow 

 motion. All that is needed for 

 this is a rack and pinion to the 

 eye-piece of considerable length. 

 An inch or two up or down cor- 

 responds here to a fraction of 

 a tui'n of the fine adjustment of 

 the Microscope, so that very 

 little danger exists of any sudden 

 contact with the cover-glass. 

 As soon as an indistinct view of the object is obtained through the 

 ordinary coarse adjustment of the microscope-body, the focus is 

 brought to exactness by means of the coarse motion of the eye-piece 

 without much difficulty. For demonstrations or exhibitions in public, 

 Microscopes could thus be made without the ordinary fine motion." 



At the June Meeting of the Society, at which Professor Ranvier's 

 plan was discussed, some objection was made to itf and we have 

 since received the further objections as follows : — 



" (1) With a high power of large aperture a slight alteration in 

 the length of the body will spoil the definition. 



" (2) There is a dif&culty in ascertaining the best focus, and the 

 eye is strained in attempting to help in focussing, particularly in 

 focussing doicn. An object should always be observed at the longest 

 focus. 



" (3) The alteration in power is apt to be misleading, in examining 

 the upper and lower planes." 



In any case we should prefer the mode of raising and depressing 

 the eye-piece adopted with English binocular Microscopes. It should 

 also be noted that to utilize the movement of the eye-piece for accurate 

 focussing, there should be a normal position from which upward and 

 downward motion can be made. 



* Journ. Quek. Micr. Club, vi. (1880) p. 165. 

 t See this Journal, ante, p. 715. 



