THE COMPOUND MICEOSCOPE. 83 



milled head, /. The other miUed head, g, fixes the arm, G, 

 to the triangular bar. 



" The outline of the structure, as before observed, has been 

 arranged to obtain, first, the utmost freedom from tremor, 

 and secondly, to afibrd the greatest facility in using the 

 various movements, 



"In experimenting to obtain the first of these conditions, I 

 suspended the moveable part of the instrument near the centre 

 of gravity, and employed the inverted pendulum (an instru- 

 ment contrived to indicate otherwise insensible vibrations) to 

 arrange the form and quantity of material so as to produce, 

 as nearly as possible, an equality of vibration throughout the 

 whole instrument; hence the object upon the stage and the 

 optical part vibrating equally, no visible vibration is caused. 

 The arrangement for accomphshing the second condition is, 

 first, that the whole movements should be as near the 

 base of the instrument as is consistent with the greatest 

 proximity among themselves ; then the milled heads, e and/, 

 for moving the triangular bar, and the fine adjustment for the 

 optical part, should be moved by the left hand, while the 

 heads, h h', for the movement of the stage, should be worked 

 by the right hand. The other milled head, e, is convenient 

 ■when the right hand may be unemployed with the stage 

 movements. The positions of the milled heads, h V, are ex- 

 tremely convenient, as the middle finger may be placed under 

 h, and the fore-finger under U, and the thumb passed from the 

 one to the other in the most natural and easy manner. The 

 left hand is also readily shifted from the milled head, e, to 

 employ the fore or middle finger to move the screw head, /. 

 This head is connected with a screw and lever, which makes 

 one revolution of it move the optical part one-three-hundredth 

 of an inch. This arrangement aflTords an elastic movement to 

 the end of the tube, as a guard against injuring the glasses or 

 the object under examination." 



In consequence of the improvements at this time being made 

 by Mr. Eoss in the illuminating part of the microscope, which 

 will considerably modify the general form of the instrument, a 

 separate description will be given in the Appendix. 

 6* 



