SELECTION OF COVERING GLASS. 215 



tion and focussed, suppose I do not get as good views 

 as I had reason to expect, then / let the collar stand as 

 it was and change the illumination until things are ap- 

 proximately as desired; this done, a slight turn of the 

 collar adjustment will insure the maximum working of 

 the objective. Now just contrast this with the usual 

 "modus." Ei^ht operators out of ten would have at 

 once twisted 'round the collar hap hazard like, by " rule 

 of thumb," probably wasting plenty of time, and, more 

 unfortunately still, condemning a really good objective, 

 and one that would have, with the proper manipulations, 

 given charming displays. 



Now, in the instance quoted, we took the mount as 

 we found it, and "handled" it as best we could with 

 the least loss of time, the only means at our command 

 being the choice between glycerine and water as the 

 immersion medium. Now suppose the mount was one 

 prepared with especial reference to a one-twenty-fifth 

 or a one-fiftieth, and having an extremely thin cover, 

 say one-four-hundredth of an inch thick. Under these 

 circumstances it would have been imperative, to suit 

 the work of the one-sixth, that this thin cover be sup- 

 plemented with another and a thicker one, making opti- 

 cal contact with water or soft balsam. This is an in- 

 convenience which, in the case alluded to, cannot be 

 well avoided ; but we are nevertheless taught the pro- 

 priety of suitably covering such mounts as may be pre- 

 pared in "our usual line of practice. All this seems plain 

 enough, without further demonstration. Let the tyro 

 then cover his mounts as far as possible to suit the 



