H. MORLAND ON MOUNTING SELECTED DIATOMS. 6 



diatom-valves are hollowed out with an edge on a flat plane, 

 these valves can be fastened down with the hollow side next the 

 slip, merely resting on the thin outer edge, and with their 

 outer surfaces uppermost and nearest the observer. This 

 obviates all clogging of fine structure by the gum, and though 

 air may be boxed in under the valve, yet as the structure of the 

 diatom-valve is generally cellular and permeable by the balsam, 

 this latter is almost always able to displace the imprisoned air 

 under the valve. Another advantage is that when a valve 

 happens to be very convex it is an exceedingly difficult affair to 

 mount such a valve squarely if resting on the convex side ; 

 moreover, as a rule, the outer surface of a diatom is far more 

 interesting to observe than an inner surface, which in many 

 forms consists simply of " eye-spots." 



The diatoms having been mounted on the slip, it is necessary 

 to so arrange that the cover-glass shall be brought down suffi- 

 ciently low as to bring the upper surface of same well within 

 the working distance of the objective. This I manage, firstly, 

 by having cover glasses not exceeding "OOo" in thickness ; and, 

 secondly, by gumming three small metal discs, '003" thick, in a 

 triangle round the "selected" diatoms, each side of the triangle 

 being about y^ of an inch. When the balsam is applied and 

 the cover-glass put on over same, and pressed down close to the 

 metal discs, it follows as a matter of simple addition that the 

 upper surface of the cover-glass is only -008" from the upper 

 surface of the slip, so that with an objective of '01" working 

 distance, the observer would be able to not only focus every 

 part of the diatom, but even beyond into the slip itself. 



So far as regards these metal discs, I may add that they are 

 of about ^^ inch in diameter, and in the first instance I pre- 

 pared them from some thin copper garden labels (Chandler's 

 patent), of which I had purchased a half -gross for trial in my 

 garden. Noticing that these 72 labels, when placed one above 

 the other, were less than ^ inch in height, it struck me that I 

 had here a material sufficiently thin to place between a slip and 

 cover, giving ample room for diatoms lying flat, and yet allow- 

 ing an objective of not more than y^ inch working distance to 

 focus down into the slip itself. My die is simply a piece of 

 cast steel, as shown in Figs. 1 and 2, 1| inches long, | inch 

 deep, and f inch thick, with a -^^ inch hole drilled through the 



