HISTORY OF CHRONOPHOTOGRAPHY. 331 



which Galileo and Atwood surniouoted to determine these laws will for 

 the future be saved in all analogous cases for those who shall employ 

 ehronophotography for the purpose. We shall only have to allow the 

 body whose motion (tig. 24, PI. Ill) is to be studied to fall before the 

 pitch-dark background, and its positions will be marked upon the sensi- 

 tive plate; the chronograph wall give the interval of time which elapses 

 between the body's arrivals at the positions figured; the scale of milli- 

 meters will measure the distances described. The same arrangement 

 enables us to make interesting studies of the resistance of the air. 



Hj'drodynamics is commonl}^ taken to be one of the most compli- 

 cated sciences. The nature of undulations, the nature of violent 

 waves, the internal motions of molecules in a shaken liquid, the man- 

 ner in which stream lines behave when they meet obstacles of different 

 forms, all these questions are still discussed as if they were difficult. 

 All these problems find their experimental solution in chronophotog- 

 raphy. 



All that is wanted is to render visible, and alone visible, before a 

 dark background, those parts of the liquid of which we wish to know 

 the motion. For that purpose into a canal formed of transparent 

 plate glass is to be poured some very clear water. A mirror inclined 

 at a convenient angle and placed under this canal reflects the light of 

 the sun, which then traverses the liquid mass from below upward. The 

 water is not illuminated; but at the surface of the water, at the point 

 where the wall of the glass is moistened b}' the liquid, a meniscus is 

 formed, and the under convex surface of this meniscus sends by total 

 reflection a very bright thread of light, which oscillates like the sur- 

 face of the liquid itself. The photographic objective will make upon 

 the sensitive plate a photograph of this line with all its movements. 



The interior of the liquid is not lighted up. In order to render cer- 

 tain points of this mass and to perceive the displacements which the}' 

 undergo it is only necessary to put into suspension in the water small 

 silvered pearls to which has been given the precise specific gravity of 

 the liquid. These pearls by the agitation w^hich they undergo will 

 express the motion of the molecules of the water at different parts of 

 the mass (fig. 25, PI. IV). 



Other phenomena of the same class can be studied bj^ chronophotog- 

 raphy. Thus, a thin inclined plane being presented to a liquid cur- 

 rent, the bright pearls will express by the direction of their course the 

 motion of liquid fillets. By the distances between their images they 

 will express the rapidity of the current. A scale of millimeters 

 immersed in the water will measure the extent of the motions, while 

 the known interval of time which separates the flashes affords the 

 means of evaluating the velocity. That having been explained, one 

 glance at the scale diagram (tig. 26, PI. IV) will suffice to show what 

 movements will take place at the surfaces of liquids under conditions 



