196 MICROSCOPIC ILLUSTRATIONS. 



body, or to the lengthening piece over which the boots 

 slide. Dissections may frequently be carried on under 

 the surface of fluids by means of these boots, with great 

 effect, which can hardly be accomplished in the common 

 way. 



Whenever the instrument is in a vertical position (as will, 

 of course, be the case in the present instance,) or when it 

 is not far off" from it, the lever y may be used with great 

 advantage. In observing live subjects it is peculiarly use- 

 ful, for it enables us to follow their movements much more 

 rapidly than can be done by means of the wheel and end- 

 less screw ; and the double motion of the arm being, by its 

 assistance, governed entirely by one hand, the other is left 

 at liberty to adjust the focus (which is, of course, perpe- 

 tually changing, according to the motion of the insects 

 observed) ; it is attached to the pinion on either side of 

 the rack-work, according to circumstances, by being 

 pushed on the square projecting pin. The wheel must 

 then be detached from the endless screw, by slackening 

 the clamping screw, 'b, and drawing the piece which 

 carries the said endless screw backwards away from the 

 wheel, causing it to traverse over a portion of the arc 

 which it forms on its pivot, and fixing it afresh, so that 

 it shall not catch the teeth of the wheel : the arm then 

 moves freely round on the top of the bar, and is subservient 

 to the action of the lever, the milled head of which operates 

 upon the pinion in place of the ordinary one, and is 

 governed by the thumb and index. finger ; while the others 



