50 MOVEMENT 



corresponds to the variations in the length of the 

 column of mercury. 



To illuminate the column of this instrument, we 

 used a series of flashes from an induction coil furnished 

 with a condenser. This intermittent illumination dis- 

 turbed the continuity of the images, and thus a series 

 of bright lines of unequal length was produced. 



By this means tracings can be obtained to demon- 

 strate the electrical changes in the hearts of tortoises 

 or frogs, as they occur respectively during the periods 

 of systole and diastole. 



The sinuous border representing the summit of the 

 column of mercury in such a tracing has a very close 

 resemblance to the curve obtained by mechanically 

 registering the actual movements of the heart during 

 its various phases. The heart, like all other muscular 

 structures, shows changes in its electrical condition, 

 according as it is contracted or relaxed. 



Determination by Means of Chronophotography of the 

 Movement executed by a Falling Body. — In the study 

 oh movement, photography has the advantage of not 

 being obliged to borrow any motive power from the 

 object observed. The following experiment may be 

 made. A black-velvet curtain may be hung vertically 

 so as to form a dark screen, in front of which a white 

 ball, lit up by the sun's rays, is allowed to fall. A 

 divided scale is placed vertically in front of the dark 

 background, to measure the distance traversed. A 

 chronometric dial is used to measure the intervals 

 between the successive images. 



When the circular diaphragm has acquired the 

 desired velocity, an assistant pulls the string and the 

 ball falls. The photographic plate receives a series of 

 images of this ball, showing the positions it occupies 

 at each successive exposure. In this way all the 

 necessary elements are obtained for determining " the 



