176 PHYSIOLOGICAL PHYSICS. [Chap. xvn. 



by the height of the line. The plate may then be 

 moved, and another tracing obtained for comparison 

 with the first. With such an arrangement a record 

 can be obtained of the varying degrees of contraction 

 by varying the stimulus, or varying the weight, or 

 varying the interval between each stimulus, and so on. 

 Again, without stimulating the muscle, the elasticity 

 of the muscle may be measured by putting weights 

 into the pan, and finding how the muscle returns to its 

 original length on removal of the weight. One may 

 note also different degrees of stretching with different 

 weights. 



To obtain a curve of muscular contraction a modi- 

 fication of the instrument was made by Du Bois- 

 Reymond. In the original form of the instrument 

 the glass plate was not so long as shown in the figure, 

 and was movable by turning a screw by hand. Instead 

 of the plate being movable in its frame by hand or 

 by screw, a spring arrangement is substituted. The 

 glass plate is long, and is movable along wires, as 

 shown in the figure. When pushed to one end, the 

 plate is held there by a catch against the force of a 

 spring wound spirally on the rod B. But as soon as 

 the catch is released, the spring causes the plate to drish 

 across from one side of the frame to the other. On the 

 support beneath the plate are two binding screws con- 

 nected by a lever. These binding screws are in the cir- 

 cuit of the primary coil of an induction machine, and 

 when the lever touches both the current passes ; when 

 the contact of the lever is broken the current is inter- 

 rupted. The muscle is connected with wires from the 

 secondary coil of the inductorium, and if the current 

 in the primary coil be interrupted, the muscle is stimu- 

 lated to contraction. Now when the liberated glass 

 plate dashes across, a piece of brass projecting from its 

 under edge knocks the lever aside, and at this instant 

 the muscle gets a shock ; the muscle being supported 



