MICROSCOPIC CHRONOPHOTOGRAPHY 293 



graphy has of late years been brought to a high 

 standard of perfection, and apparently at the present 

 time it is only necessary to follow the rules laid down 

 i:i the various treatises on the subject. In the practical 

 application, however, difficulties arise the causes of 

 which are not far to seek. They consist principally 

 in the illumination of the object. 



Very small organisms generally move at a rate 

 quite disproportionate to their size. Infusoria cross 

 the field of the microscope in a moment, and execute 

 an immense number of movements which the eye 

 cannot follow. The vibrating cilia, for instance, which 

 serve as locomotor appendages in many of these 

 animalcules, vibrate with such rapidity that they are 

 absolutely invisible, and only come into view when 

 the animals are dead. 



To obtain, therefore, distinct photographs of these 

 movements, the exposure must be extremely short, 

 We have already learned that, to obtain photographs 

 of the wings of an insect during flight, the exposure 

 must be reduced to gsooo* P art oi a seconc ^ an( ^ that, 

 too, under conditions of brilliant illumination ; in 

 fact the insects were photographed in silhouette 

 against the disc of the sun itself. For microscopic 

 creatures, even this illumination would be altogether 

 inadequate, in view of the extremely short exposure 

 necessitated by the rapidity of their movements. Ft 

 is no longer possible to photograph the object to 

 actual scale, it must be enormously magnified, and 

 this magnification entails a corresponding diminution 

 of the light which reaches the sensitized plate. 



A linear magnification of 100 diameters reduces 

 ten thousand-fold the intensity of the light dis- 

 tributed over the plate. 



It is true that with powerful lenses the solar rays 

 can be sufficiently condensed, but then the heat which 



