OF MICROSCOPIC OBJECTS. do 
or covered with black paper, on which the object is fixed by 
gum or some other adhesive substance. They are usually 
pierced longitudinally by a strong pin, which serves for the 
forceps to lay hold of when being placed under the micro- 
scope for examination. Sometimes objects are affixed to 
both sides of the disc, which is readily turned when under 
the object-glass. The advantage of this method of mount- 
ing is the ease with which the disc may be moved, and so 
present every part of the object to the eye, save that by 
which it is fastened to the disc. On this account it is often 
used when some particular subject is undergoing investiga- 
tion, as a number of specimens may be placed upon the 
discs with very little labour, displaying all their parts. But 
where exposure to the atmosphere or small particles of dust 
will injure an object, no advantage which discs may possess 
should be considered, and an ordinary covered cell should 
be employed. Small pill-boxes have been used, to the bot- 
tom of which a piece of cork has been glued to afford a 
ground for the pin or other mode of attachment; but this 
is liable to some of the same faults as the disc, and it would 
be unwise to use these for permanent objects. 
Messrs. Smith and Beck have lately invented, and are 
now making a beautiful small apparatus, by means of 
which the disc supporting the object can be worked with 
little or no trouble into any position that may prove most 
convenient, whilst a perforated cylinder serves for the recep- 
tion of the discs when out of use, and fits into a case to 
protect them from dust. A pair of forceps is made for the 
express purpose of removing them from the case and placing 
them in the holder. 
All dry objects, however, which are to be preserved should 
be mounted on glass slides in one of the cells (described in 
Chapter II.) best suited to them. Where the object is to be 
free from pressure, care must be taken that the cell is deep 
enough to ensure this. When the depth required is but 
small, it is often sufficient to omit the card, leatker, or 
other circles, and with the “turntable” before described by 
