164 



THE SIGNS OF LIFE 



On the screen, at a distance of 8 to 12 feet the magnification of 

 the image given by the -^Q inch objective is 1000 to 1500 diameters, 

 z'.., the column has an apparent diameter of 25 to 37.5 mm. an 

 inch to an inch and a half. On the photographic plate, at a 

 distance of 2 feet, the apparent diameter is upwards of 6 mm. 

 far more than is sufficient to certainly cover the vertical slit, the 

 breadth of which is less than | mm. For the frog's heart, a lower 

 magnification is generally sufficient. For the human heart, as 

 high a magnification as practicable should be employed. 



o Sec. -I -z --5 -4- -5 -6 -7 



miiiivoit 

 0-5 



i 



C-5\ 



\ I 



\ I 



\ I 



S/ 



kb ha.nd 

 to H\ 



I-O\ 



/5 1 



FlG. 66. An electrometer-curve of the electrical variation of the human heart ; 

 the values calculated from the record of the mercury level are given as a dotted line. 

 (From Einthoven, Pfliiger's Archni, vol. 60, 1895.) 



The principle upon which electrometer values are obtained from 

 such electroscopic records is readily illustrated by demonstrating the 



