24:^ 



If a strong magnitioaliou were used with this urrangemeiit witlioiü 

 any further arrangements the images in the field of projection would 

 fall so far apart that tiie apparatus would he useless in practice. 

 With a magnitication of 600, the images of the strings would lie 

 30 cm. apart. To avoid liiis difficulty, the rays which arc dii'ected 

 upon tiie slit by the projectiou-ocuhvr. are changed in direction by 

 a pair of achromatic prisms in sucli a manner, that the images of 

 the strings come to be at a convenient distance from one another 

 upon the slit. A rectangular screen, placed at some distance in front 

 of the slit, divides the two fields formed by the prisms, and forms 

 a fine line of shadow upon the sensitive plate. 



C. The third method of combination of galvanometers may 

 perhaps be called the most elegant, but it demands a \ery careful 

 adjustment. The principle of this method is that the two galvano- 

 meters are placed one behind tiie other, with the optical axes falling 

 in the .same line. 



Midway between the projection-objective of the fir.st galvanometer 

 and the illumination-objective of the secoud a combination of lenses 

 is introduced which may be compared to a double ocular, and 

 which the firm of Cakt, Zkiss have been kind enough to construct 

 at our recpiest. This system is placed at such a distance from the 

 two above mentioned objectives, that tlie spherical and chromatic 

 aberrations of the image are eom|)eusated as well as possible. 



The string which is nearest to the lamp is first projected inthe 

 new ZEiss-system, a second liuie in llio oplical field of" the second 

 string; a third time in the |)rojection-()culai', and finally a fourth 

 time upon the sensitive plate. 



Although great demands are made upon the optical apparatus in 

 order to insure shai'pucss in this fourth image, yet the curves show 

 that the method leads to very satisfactory results. The images are 

 so sharp and full of contrast, that it is sometimes almost impossible 

 to distinguish between the image of the first and that of the second 

 string. This may be seen for instance in the curves in the thesis 

 of Dr. Battaerd, in which heart sounds and E.K.G. were simulta- 

 neously registered by the method in (|uestion. 



In applying the method of simultaneous registration to electro- 

 cardiography special precautions must be taken. In this paper we 

 discuss the use of three galvanometers at once. 



If an E.K.G. is made with only one derivation, regulating the 

 sensitivity of the galvanometer in the usual way, each centimetre 

 of an ordinate of the curve represents a potential difference of 1 



