120 



THE MICROSCOPE AND ITS REVELATIONS. 



they are deflected towards its axis; but as their angle of convergence is 

 less than the angle of divergence of the rays proceeding from the object, 

 the reflected rays will not meet in the focal point of the lens, but will be 

 so distributed as to illuminate a sufficient area. By altering the extent 

 to which the prism is pushed in, or by lifting or depressing its outer end 

 by means of a milled-head screw, the field of illumina- 

 tion can be regulated. The working of this prism with 

 immersion objectives is stated by Mr. Tolles to be pecu- 

 liarly satisfactory. 1 



117. Stepliensori* s Safety Stage. In examining ob- 

 jects with those higher powers which focus extremely 

 close to the covering glass, the slightest inadvertence 

 is likely to lead to a fracture of the glass, and perhaps 

 to the destruction of a valuable slide. This is a serious 

 matter with Moller's Diatom Type Slide, or Nobert's 

 Test Lines, or with many others that are expensive or 

 perhaps impossible to replace. To remove this source 

 of danger, Mr. Stephenson contrived the "safety stage," 

 shown in Fig. 92. The frame on which the slide car- 

 Safety-stage. r yi n g the object rests, is hinged at its upper part, and 

 kept in its true position by slight springs, which give 

 way directly the slide is pressed by the objective. It is found that springs 

 firm enough to insure the steadiness required for high powers, may yet 

 be sufficiently flexible to give way before very thin glass is endangered, 

 and a glance at the stage shows if it is made to deviate from the nor- 

 mal position in which its upper and lower edges are parallel. (See also 

 54 



Section 2. Apparatus for the Presentation of Objects. 



118. Stage-Forceps and Vice. For bringing under' the Object-glass 

 in different positions such small opaque objects as can be conveniently 

 held in a pair of forceps, the Stage-Forceps (Fig. 93) supplied with most 



Microscopes afford a ready 

 means. These are mounted by 

 means of a joint upon a pin, 

 which fits into a hole either in 

 the corner of the Stage itself or 

 in the object-platform; the ob- 

 inserted by pressing the 



stage-Forceps. 



pin that projects from one of the blades, whereby it is separated from the 

 other; and the blades close again by their own elasticity, so as to retain 

 the object when the pressure* is withdrawn. By sliding the wire stem 

 which bears the Forceps through its socket, and by moving that socket 

 vertically upon its joint, and the joint horizontally upon the pin, the 

 object may be brought into the field precisely in the position required; 

 and it may be turned round and round, so that all sides of it may be 

 examined, by simply giving a twisting movement to the wire stem. The 

 other extremity of the stem often bears a small brass box filled with cork, 

 and perforated with holes in its side; this affords a secure hold to 

 commmon pins, to the heads of which small objects can be attached by 

 gum, or to which disks of card, etc., may be attached, whereon objects 

 are mounted for being viewed with the Lieberkuhn ( 115). This 



1 " Journ. of Roy. Microsc. Soc.," Vol. iii., pp. 526, 754. 



