o2 PREPARATION AND MOUNTING 
of the objects is inconsiderable, especially with some of the 
Diatomaceze, often termed “ test-objects.” The wooden: 
slide is cut with the central opening as above, and two 
pieces of thin glass are laid upon it, betwixt which the 
diatoms or other objects are placed, and kept in their proper 
position by a paper cover. This arrangement is a good 
one, insomuch as the very small portion of glass through 
which the light passes on its way to the microscope from 
the reflector causes the refraction or interference to be 
reduced to the lowest point. 
A novice would naturally think the appearance of some 
cof the slides above mentioned very slovenly and unfinished ; 
but they are often covered with ornamental papers, which 
may be procured at almost every optician’s, at a cost 
little more than nominal, and of innumerable patterns and 
colours. How to use these will be described in another 
place. 
It is very probable that a beginner would ask his friend 
what kind of slides he would advise him to use. Almost 
all those made of wood are liable to warp more or less, even 
when the two pieces are separate or of different kinds; those 
of cardboard and wood are generally free from this fault, 
yet the slides, being opaque, prevent the employment of 
the Lieberkuhn. ‘To some extent glass sides, when covered 
with ornamental papers, are liable to the same objection, 
as the light is partly hindered. And sometimes dampness 
from the paste, or other substance used to affix the papers, 
penetrates to the object, and so spoils it, though this may 
be rendered less frequent by first attaching the thin glass to 
the slide by some harder cement. Much time, however, 
is taken up by the labour of covering the slides, which is a 
matter of consideration with some. Certainly the cost of 
the glass slides was formerly great; but now they are 
reasonable enough in this respect, so that this objection is 
removed. It is, therefore, well to use glass slides, except 
where the thin glasses are employed for tests, &c., as above. 
When the thin glass circles are placed upon the slides, and 
