GALVANOTROPISM OF TADPOLES. 1 



GENTLEMEN, you have before you on the screen 

 an image of a tadpole in a rectangular trough of 

 water through which a galvanic current will be passed via 

 two flat metallic surfaces forming the two short ends of the 

 trough. Just above is the image of a galvanometric pointer, 

 the movements of which will indicate the direction and 

 strength of current. A commutator in circuit varies the 

 direction of current, and a resistance that can be altered 

 by turning a screw {i.e., an Engelmann's screw-rheostat) 

 serves to vary its strength. The curved scale projected 

 with the pointer upon the screen is graduated in milli- 

 amperes. 



It is not enough to say that so many milliamperes of 

 current are used, we must also — in order to have some idea 

 of what passes through the tadpole — know what is the 

 "current-density" in the water. Obviously a current of 

 say one milliampere will be of less density if it is passing 

 through a wide than through a narrow column of water 



1 A Lecture in Electro-Physiology delivered by Dr. Augustus Waller, 

 M.D., F.R.S., at his private laboratory, in June, 1895. — Editor. 



