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 be attained more advantageously by means of a 

 quadrangular prism, as shown in fig. IS. 



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. IH 

 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 SUPPORT AKD 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 b^ 

 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 



