THE MICROSCOPE. 41 



Place on the stage of the instrument, as before, a known 

 divided scale, and when it is distinctly seen, hold a rule at ten 

 inches distance from the disengaged eye, so that it may be seen 

 by that eye, overlapping or lying by side of the magnified pic- 

 ture of the other scale. Then move the rule till one or more 

 of its known divisions correspond with a number of those in 

 the magnified scale, and a comparison of the two gives the 

 magnifying power. 



Having now explained the optical principles of the achromatic 

 compound microscope, it remains only to describe the mechan- 

 ical arrangements for giving those principles their full effect. 

 The mechanism of a microscope is of much more importance 

 than might be imagined by those who have not studied the sub- 

 ject. In the first place, steadiness, or freedom from vibration, 

 and most particularly freedom from any vibrations which are not 

 equally communicated to the object under examination, and 

 to the lenses by which it is viewed, is a point of the ut- 

 most consequence. When, for instance, the body contain- 

 ing the lenses is screwed by its lower extremity to a horizon- 

 tal 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 necessarily 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 com- 

 paratively insensible degree, the image is seen to oscillate 

 so rapidly, as in some cases to be wholly undistinguishable. 

 Such microscopes cannot possibly be used with high pow- 

 ers in ordinary houses abutting on any paved streets through 

 which carriages are passing, nor indeed are they adapted to 

 be used in houses in which the ordinary internal sources of 

 shaking exist. 



One of the best modes of mounting a compound microscope 

 is shown in the annexed view (Fig. 22), which, though too min- 

 ute to exhibit all the details, will serve to explain the chief fea- 

 tures of the arrangement. 



A massy pillar A is screwed into a solid tripod B, and is sur- 

 mounted by a strong joint at C, on which the whole instru- 

 ment turns, so as to enable it to take a perfectly horizontal or 



