26 CONSTRUCTION 



exactly as has been described in reference 

 to the single microscope and to the com- 

 pound of two glasses. 



17. Having thus briefly explained the 

 optical principles of the achromatic com- 

 pound microscope, it remains only to de- 

 scribe the mechanical arrangements for 

 giving those principles effect. The mechan- 

 ism of a microscope is of much more im- 

 portance than might be imagined by those 

 who have not studied the subject. In the 

 first place, steadiness or freedom from vi- 

 bration, and most particularly steadiness 

 or freedom from any vibrations which 

 are not equally communicated to the ob- 

 ject under examination, and to the lenses 

 by which it is viewed, is a point of the 

 utmost consequence. When, for instance, 

 the body containing the lenses is screwed 

 by its lowest extremity to a horizontal 

 arm, we have one of the most vibratory 

 forms conceivable ; it is precisely the form 

 of the inverted pendulum, which is ex- 

 pressly contrived to indicate otherwise 

 insensible vibrations. The tremor neces- 

 sarily attendant on such an arrangement, 

 is magnified by the whole power of the 

 instrument ; and as the object on the stage 

 partakes of this tremor in a comparatively 

 insensible degree, the image is seen to 

 oscillate so rapidly, as in some cases to be 

 wholly undistinguishable. 



One of the best modes of mounting 

 a microscope is shown in the annexed 

 plate, by a reference to which the reader 

 will be enabled to understand the chief 

 features of the arrangements. (Fig. 12.) 



