238 OPTICAL PROJECTION 



ventricle of a heart and the other on the auricle. When the 

 auricle is compressed the slab sinks, and so does the mirror, 

 as shown by the dotted lines ; whilst the slab on the ventricle 

 rises and the other mirror is elevated. The optical arrange- 

 ments are as in fig. 124. The focussed parallel pencils from 

 two small apertures on the lantern B fall on the mirrors of 

 the cardioscope c, and are reflected and focussed on the screen 

 T. The spots move in harmony, and one of course represents 

 the systole and the other the diastole of the heart, as the 

 mirrors rise and fall. 



Contraction of Muscle, which it has already been remarked 

 may be projected by the apparatus shown in fig. 122, may be 



also demonstrated on the 

 screen by the telegraph of 

 Du Bois-Reymond, the general 

 scheme of which will be seen 

 in fig. 125. On the base gg' 

 the pillar D is fixed, and sup- 

 ports a pair of forceps A, which 

 can slide in and out of the 

 tubular socket or handle B, 

 FIQ. 125. Muscle Telegraph being fixed at any length by 



the screw s. The other pillar 



can be fixed at a movable distance on the base by the screw z. 

 This pillar carries a little pulley from which projects a radial 

 arm a 1 carrying at the end the index-disc d. Over the pulley 

 passes a thread, which carries at the end a small capsule or 

 bucket b for weighting, and at the other end has a hook x. 

 One 'end of a frog's muscle properly prepared with its nerve 

 is held in the forceps A, and the hook is passed through the 

 tendo Achillis, the distances are properly adjusted, and the 

 bucket weighted with small shot, so that the slightest con- 

 traction pulls the thread and raises the index-disc in the 

 direction of the arrow. By binding screws at s' and x wires 

 can be connected for applying a galvanic current. By this 



