OF MICROSCOPIC OBJECTS. 53 



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 far 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 

 email, it is often sufficient to omit the card, leather, or 

 other -circles, and with the " turntable " before described by 



