H. NORLAND ON MOUNTING DIATOMACEjE. 321 



that may have formed under the cover-glass. I may here say 

 that, when once the benzole has been driven off, styrax mounts 

 can stand, and really require a considerable amount of long-con- 

 tinued heat to harden them off properly, and, unless these 

 mounts are over-heated, the styrax never really becomes hard, 

 but only tough, so that I make it a rule to finish off all my 

 styrax mounts with a ring of either Bell's cement or Hollis's 

 liquid glue. 



I fancy I can hear many exclaim, " Why, what is this ? 

 There is no occasion for this length of time over a balsam 

 mount. I can finish one off in an hour, or even less." Perhaps 

 so, but will such a person guarantee perfect mounts — no bubbles 

 in frustules or near the edges of cover-glass r Besides which, 

 if the medium be boiled even in the slightest degree it will 

 follow as a matter of course that a larger number of diatoms 

 will be detached from the underside of cover-glass than if no 

 boiling whatever had taken place, and I think that all will 

 allow that it is desirable to have as many diatoms on the cover- 

 glass as possible, especially when we come to examine such 

 slides under a short distance working object-glass. It is 

 advisable to have sufficient of the medium under the cover - 

 glass lest the same should sink down too low and crush the 

 diatoms under it; if, therefore, to avoid this mishap a little too 

 much balsam be used, it may happen that when a diatom falls 

 away from the cover-glass down on to the slip beneath, the 

 same may be beyond the focus of a high power objective. 

 But granted even that the objective has sufficient working 

 distance to examine down to the surface of the slip, it is still 

 advisable that by far the greater majority of the diatoms 

 should be on one focal plane, as otherwise the constant focusing 

 up and down, whilst looking over a " spread"' slide, will be 

 found to be very trying to the eyes. Moreover, with every care, 

 some of the smaller diatoms can be easily overlooked, even when 

 immediately under the objective, in consequence of being either 

 within or without the focus of same. 



I will now r proceed to the description of how I mount 

 " selected " slides, but before doing so I wish to say that 1 can 

 give no certain and sure method of preparing such slides, by 

 this I mean slides that can be placed under a -^ inch objective 

 without flinching, as such high powers will only too often show 



