BY DR. MICHAEL FOSTER. 357 



and not on its vertical, axis, the simple lever must be fixed in 

 a horizontal position at the front of the platform, and the silk 

 brought in a straight line to it from the tendon. Resistance 

 to the action of the muscle may then be gained by means of 

 the light spring, or a weight passing over another pulley. As 

 a rule, it is best, when possible, to, do without a pulley. 



XII. Recording Tuning-Fork. For measuring small 

 intervals of time in physiological observations, it becomes 

 absolutely necessary to make use of tuning-forks of known 

 rates of vibration. Fig. 277 is a figure of a tuning-fork 

 arranged by Konig for recording its vibrations on a revolv- 

 ing surface. A massive stand bears the fork A firmly secured 

 in it. The two coils c c' (which by means of the arrangement 

 k can be slid up and down the stand, so as to accommodate 

 themselves to tuning-forks of different lengths) project over 

 the two ends of the fork, and each bears a screw d which can 

 be screwed as desired up to or away from the ends of the fork. 

 The upper arm of the fork bears at its end a rod a with plati- 

 num point which dips into the mercury cup b. 



The tuning-fork B, which must have the same rate of vibra- 

 tion as A, is fixed into a light movable stand, so that it can 

 be placed in such a position that the light elastic marker g 

 may touch with the least excess of friction the recording sur- 

 face. This fork is" placed in the same manner as A, with its 

 ends between the coils e e', bearing similarly the screws f f . 



One wire from a battery w is connected witli a binding 



^ screw at the handle of the tuning-fork A, and is thus in electric 



continuity with the rod a. The mercury cup b is connected by 



a wire z with the coil e of the fork B. The other pole of the 



v/ battery is connected by x with the coils c c' of A, and thence 



by y with the coil e f of B. 



The screws d d, ff being brought rather near to their re- 

 spective forks, place a small quantity of mercury covered by a 

 little spirit in the cup 6, and having set the fork A vibrating a 

 little, screw the rod a up or down until magnetic interruption 

 is fairly established. B will then be found vibrating synchro- 

 nously with A, and the point g will be found tracing curves 

 on the recording surface, the interval of time corresponding 

 to each curve being determined by the pitch of the fork. 

 Screw the d d, //', as far away from their respective forks as 

 can be done without stopping the current altogether. 



XIII. Arrangement of Electrical Apparatus. Con- 

 stant Current. Place the nerve (or muscle, when muscle 

 alone is the subject of experiment) on the electrodes, taking 

 care that the nerve is actually in contact with each electrode. 

 When the non-polarizable electrodes are used, their plugs must 

 be kept damp with the normal saline solution: avoid making 



