302 MOVEMENT 



opening, and reappear upon the screen, the knob must 

 no longer be pressed, so that the process of taking 

 photographs may cease. When the creatures are 

 again noticed to be in a favourable position, the opera- 

 tion may be continued. 



There are many advantages in this method ; firstly, 

 it enables the operator to focus more accurately, because 

 he can directly gauge the definition of the image at 

 the moment of exposure ; and, secondly, it allows him 

 to avail himself of fugitive phenomena which would 

 otherwise be lost, especially if it is necessary to 

 make rather complicated arrangements before exposing 

 the plate ; and, finally, it protects him from injurious 

 exposure to light, in case it should fall upon the retina, 

 a danger which is threatened in other methods, because 

 the light which is thrown by the heliostat, and con- 

 centrated by the condenser, may by chance traverse 

 the tube of the microscope. 



In this way we obtained photographs of infusoria in 

 motion, the contraction of the internal organs of certain 

 larvae, the action of the limbs and prehensile appen- 

 dages of all kinds of microscopic creatures. There 

 even seems a possibility of discovering some curious 

 facts as to muscular contraction from observing certain 

 transparent larvae. 



This method is also well adapted for studying the 

 crystallization of different salts. If a saline solution 

 be concentrated by means of evaporation, crystals 

 make their appearance. There is a special form for 

 each salt, and from these centres of crystallization a 

 variety of arborizations radiate in all directions, and 

 invade the microscopic field with unequal rapidity. 

 The formation of these crystals can be well seen on 

 the screen, and as soon as the edge of the crystalline 

 development is noticed to spread over the admission 

 aperture the photographs can be taken. Each image 



