22 



High Power Microscopy. 



[Feb. 17, 



as it is at rest. By keeping it in motion it can be rendered invisible, 

 and the motion need not be rapid for, like the grain itself, the move- 

 ment is magnified by the npper microscope through which it is 

 viewed. But two precautions must be observed. The screen must 

 be kept accurately in the focal plane and it must not move in a 

 closed path. The first is obvious and it will appear upon a moment's 

 reflection that if the bright points upon the screen described closed 

 paths in the field of view those paths would be delineated in the 

 picture by bright lines. 



The contrivance is illustrated in Fig. 10 by which in this piece 

 of apparatus the necessary oscillation in a path too complicated to be 

 followed by the eye or traceable on a photographic 

 plate is imparted to the screen. Here a pulley re- 

 volves upon a stud, the stud being perforated to 

 allow the image-forming beam to pass. This fora- 

 men in the stud occupies in use the exact centre of 

 the tube of the instrument. The top of the stud is 

 flattened and forms a platform upon which the 

 ground-glass screen rests. Upon the upper face of 

 the pulley a ring is mounted eccentrically, and in the 

 hollow of this circular ring the screen lies ; fitting it 

 very loosely. Now when the pulley rotates it will 

 of course rotate the eccentric ring, and the ring, 

 wobbling round, will drag the screen round with it 

 in its eccentric revolution. But the screen, being 

 loose within the ring and resting on the stationary 

 stud, will tend to lag behind the ring in its move- 

 ment and will roll upon its edge within the ring with a relatively 

 backward motion. The whole result is that the screen oscillates and 

 revolves a])out a constantly varying centre of motion, and the paths 

 described by its various parts do not return into themselves. 



By this simple expedient the whole difficulty is overcome. The 

 screen abolishes the intrusive images of the dust and foreign matter ; 

 the motion renders the screen itself invisible. Fig. 11 is a photo- 

 graph of the image which with these precautions can be obtained 

 even under such extreme conditions of super-amplification as those 

 above described. And the image so formed is just as perfect when 

 viewed directly as when recorded by the aid of photography. 



Fig. 10. 



[J. W. G.] 



