THE STAGE. 
it is sometimes preferable to accomplish the object by two suc¬ 
cessive reflections, as shown in fig. 17, where the ray, a b, is 
successively reflected at b and c to the eye at d. And the same 
object may b^ f attained more advantageously by means of a 
quadrangular prism, as shown in fig. 18. 
This application of the prism and reflector has been already 
explained in our Tract upon Optical Images. 
Much practical convenience often arises from the adoption of 
this expedient; thus, while the object-tube is directed vertically 
downwards, to an object supported on a horizontal stage, or 
floating on or swimming in a liquid, the eye-tube may be hori¬ 
zontal, so that the observer may look in the level direction. In 
this case the two tubes are fixed at right angles, the reflecting 
surface being placed at an angle of 45° with their axes. We 
shall see hereafter a case in which, by the adoption of an oblique 
tube, several observers may at the same time, looking through 
different eye-pieces, see the same object through one and the 
same object-glass. 
THE STJTPOET AND MOVEMENT OF THE OBJECT. 
31. The appendage of the microscope, adapted for the support 
of the object is called the stage. 
Since every motion or disturbance by which the stage may be 
affected will necessarily be increased, when seen through the 
microscope, in the exact proportion of the magnifying power, it 
is of the utmost importance that it should be exempt from all 
tremor, and that it should have strength sufficient to bear, with¬ 
out flexure, the pressure of the hands in the manipulation of the 
object. When a high power is used, the focal adjustment of 
the instrument requires to be so exact, that a displacement of 
the object, which would be produced by the slightest pressure 
of the fingers upon a stage not very firmly supported, would 
throw it out of focus. 
If the instrument be used for dissection, or any other purpose 
in which steady manipulation of the object is needed, it will be 
found convenient that the stage have sufficient magnitude to 
support both wrists, while the operation is performed with the 
fingers. Supports for the elbows ought also to be arranged, so as 
to place the operator completely at ease. 
32. The instrument is focussed, as already explained, either by 
moving the stage to and from the body, or by moving the body 
to and from the stage. The motion is imparted to the one or the 
other by means of a milled head placed on the right of the 
observer, which leaves a pinion working in a rack to which the 
d 2 35 
