The Microscope. 279 



ment to examine this model closely. Mr. Walmsley will send 

 circulars to all who request it. 



SELECTIONS. 



MICROSCOPICAL EXAMINATION OF CILIATED OR- 

 GANISMS BY INTERMITTENT LIGHT. 



BY GEORGE M. HOPKINS. 



Every observing person has noticed that moving objects 

 appear stationary when viewed by a flash of light; examples 

 of this are seen during every thunder storm occurring in the 

 night. The wheels of a carriage, a moving animal, or any mov- 

 ing thing, seen by the light of the lightning, appears perfectly 

 stationary, the duration of the light being so brief as to admit 

 of only an inappreciable movement of the body while illumin- 

 ation lasts. 



If by any means a regular succession of light flashes be 

 produced, the moving body will be seen in as many different 

 positions as there are flashes of light. If a body rotating rap- 

 idly on a fixed axis be viewed by light flashes occurring once 

 during each revolution of the body, only one image will be ob- 

 served, and this will result from a succession of impressions 

 upon the retina, which by the persistence of vision become 

 blended into one continuous image. In this case no movement 

 of the body will be apparent; but if the flashes of light suc- 

 ceed each other ever so little slower than the rotatory period of 

 the revolving body, the body will appear to move slowly for- 

 ward, while in reality it is moving rapidly ; and should the light 

 flashes succeed each other more rapidly than the revolutions of 

 the revolving body, the body will appear to move slowly back- 

 ward, or in a direction opposite to that in which it is really 

 turning. These curious effects are also produced when the num- 

 ber of the light flashes is a multiple of the number of revolu- 

 tions, or vice versa. 



The combined effect of interrupted illumination and per- 

 sistence of vision may be practically utilized for examining ob- 

 jects under motion which could not otherwise be satisfactorily 



