CHAPTER II. 



CONSTKUCTION OF THE MICROSCOPE. 



25. The optical principles whereon the operation of the Micro- 

 scope depends, having now been explained, we have next to 

 consider the mechanical provisions, whereby they are brought to 

 bear upon the different purposes which the instrument is 

 destined to serve. And first it will be desirable to state those 

 general piinciples, which have received the sanction of universal 

 experience, in regard to the best arrangement of its constituent 

 parts. Every complete Microscope, whether Simple or Compound, 

 must possess, in addition to the lens or combination of lenses 

 which affords its magnifying power, a stage whereon the object 

 may securely rest, a concave min-or for the illumination of trans- 

 parent objects from beneath, and a condensing-lens for the illumi- 

 nation of opaque objects from above. 



I. Now in whatever mode these may be connected with each 

 other, it is essential that the optical part and the stage should he so 

 disposed, as either to be altogether free from tendency to vibration, or 

 to vibrate together; since it is obvious that any movement of one, 

 in which the other does not partake, will be augmented to the 

 eye of the observer in proportion to the magnifying power 

 employed. In a badly-constructed instrument, even though 

 placed upon a steady table resting upon the firm floor of a well- 

 built house, when high powers are used, the object is seen to 

 oscillate so rapidly at the slightest tremor, such as that caused by 

 a person walking across the room, or by a carriage rolling by in 

 the street, as to be frequently almost indistinguishable : whereas 

 in a well-constructed microscope, scarcely any perceptible effect 

 will be produced by even greater disturbances. 



II. The next requisite is a capability of accurate adjustment to 

 every variety of focal distance, without movement of the object. It is 

 now a principle almost universally recognized in the construction 

 of good Microscopes, that the stage whereon the object is placed 

 should be a fixture; the movement by which the focus is to be 

 adjusted, being effected in the lenses or optical portion. Several 

 reasons concur to establish this principle ; of which one of the 

 most important is, that, if the stage be made the movable part, 



