OF MICROSCOPIC OBJECTS. 63 
if necessary, without being shaken or interfered with, other- 
wise uniformity of dispersion will be prevented. When 
thoroughly dry, a thin ring of one of the adhesive varnishes 
—dgold-size will be found as good as any—may be drawn 
round the diatoms, and allowed to dry in a slight degree. 
The slide and thin glass cover should then be warmed and 
the latter gently pressed upon the ring of varnish until the 
adhesion all round is complete. 
As some of the diatoms require object-glasses of extremely 
high power, and consequently short focus, to show them, 
they must be as close to the outer surface of the cover as 
possible. For this reason they are sometimes placed upon 
the under side of the thin glass, as follows. Clean the 
surfaces of the slide and cover, and with the rod or pipe 
place the liquid containing the diatoms upon the thin glass, 
and dry as before. Trace the ring to receive the cover upon 
tne slide, and when almost dry, warm both and proceed as 
above. Whichever of these methods is employed, the outer 
ring of coloured varnish may be applied as elsewhere described 
and the slide finished. 
Diatoms are also sometimes mounted betwixt two thin 
classes, as described in Chapter II., so that the light by 
which they are examined may receive as little interference as 
possible, and that an achromatic condenser may be brought 
into focus under the slide. 
Of the various modes of cleaning and mounting Diato- 
mace, I believe that the following may be safely recom- 
mended, as affordiug results of the best quality. My friend, 
Mr. T. G. Rylands, gave it to me as that which he prefers, 
and I can safely say that his numerous slides are at least 
equal to any I have ever seen. I will give it just as I 
received it from him, though there may be some little 
repetition of what has been said elsewhere, as he does not 
appropriate any part of it as his own. He says:—In this 
branch of mounting, general rules alone can be laid down, 
because the gatherings may contain iron, lime, fine silt, or 
vegetable matter under conditions for special treatment, and 
