158 THE AMERICAN MONTHLY [May, 



to the forms under observation, yet, with greater ease and 

 accuracy than by the use of the ordinary hand stage. These 

 two kinds of motions are constantly at the microscopist's im- 

 mediate disposal, by simply grasping the knob of the guide 

 between his thumb and forefinger. 



Turning our attention now to the drawings, we see that part 

 of the microscope stage, a a, to which the object guide is im- 

 mediately attached. This is done by the T piece t, provided 

 with an upright pillar x, serving as an axis to the grooved 

 block c, and representing the motion centre of the instrument. 

 There is a spring between t and c. Sliding in the partially 

 leather-lined grooves of c is the forked, or fenestrated plate z z, 

 and linked to this by the screws b b, the plate p, provided with 

 spring clips and a knob k ; c having received the plate z, is by 

 means of s s adjusted at such a height above t that p lies par- 

 allel to the stage a a. It is to be noted that plate z, by means 

 of the grooved block c and the vertical axis x, invariably 

 — whether at rest, or in motion — occupies a position in a hor- 

 izontal plane, corresponding to that of a a. Connected with z, 

 a vertical post r is seen, lodging a small bolt f which carries an 

 arched compound spring d on the left, and a milled screwnut e 

 on the right side of r. The arched spring with either of its ex- 

 tremities acts on an upright, w w, forming part of plate p, z 

 and p being connected by a hinge joint, and z being confined to 

 a horizontal plane, the action of d results in a pressure exerted 

 by p on the stage of the microscope, and by z in the grooves of 

 c. At the moving of the instrume at this pressure causes fric- 

 tion, the requisite amount of which is secured by regulating the 

 tension of d by means of e. Comparison shows, and it is clear 

 without further explanation, that the friction in the grooves of 

 c is greatly in excess of that on the stage of the microscope, 

 and that owing to this fact, the operator — holding k loosely be- 

 tween his fingers — is enabled to move the guide to and fro 

 without producing any lateral motion. It requires a somewhat 

 firmer grasp of k to move the guide lateraly through the field of 

 vision, followed again by a motion to and fro, and soon, altern 

 ately. 



In morphological studies the operator pays no attention to 

 the ruling friction in c, but simply follows the outlines of the 



