42 CONSTRUCTION OF THE MICROSCOPE. 



CHAPTER II. 



CONSTRUCTION OP THE MICROSCOPE. 



32. The Optical principles whereon the operation of the Micro- 

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

 sider 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 prin- 

 ciples which have now 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 Mirror for the illumination of Transparent objects from 

 beneath, and a Condensing-lens for the illumination 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 be 

 so disposed as either to be altogether free from tendency to vibra- 

 tion, 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 pro- 

 duced by even greater disturbances. Hence, in the choice of a 

 Microscope, it should always be subjected to this test, and should 

 be unhesitatingly rejected if the result be unfavourable. If the 

 instrument should be found free from fault when thus tested with 

 high powers, its steadiness with low powers may be assumed ; but, 

 on the other hand, though a Microscope may give an image free 

 from perceptible tremor when the lower powers only are employed, 

 it may be quite unfit for use with the higher. 



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



