PHYSIOLOGICAL DEMONSTRATION 233 



as in fig. 120. A disc of microscopic cover-glass silvered, 

 about | inch diameter, should have attached to the back two 

 morsels of beeswax and one of cobbler's wax, and be so placed 

 that the latter adheres to the spot on the wrist found to give 

 the best pulse. A parallel beam, or preferably the focussed 

 parallel beam from a small aperture in the pencil attachment 

 (fig. 95), is reflected from the mirror A to the screen, and the 

 reflected ray exhibits every beat by the oscillation of the bright 

 spot. 



This reflection is too simple to exhibit any characteristic 

 pulse-trace, as taken by the sphygmograph. Some approxi- 

 mation to it can however be obtained by receiving the reflection 

 from A upon a tilting or rocking mirror, capable of giving a 

 motion to the spot >A 

 of light at right \\ 

 angles to that given 

 by the pulse-motion. 

 Then this rocking 

 motion should be 



given, backwardsand FlG 120 ._ Pulse Mirror 



forwards, so that the 



motion synchronises with the pulse-motion ; when the break 

 in the curve will be seen very fairly represented on the screen. 



It is true that this break can only be followed for one 

 pulse at a time, and not for many beats together as in a 

 tracing. But there is a general remark to be made here re- 

 specting the true place and value in demonstration of direct 

 projections. Exhibited thus, however roughly, they give a 

 sense of vivid objective reality which can be imparted to a 

 class in no other way, to ' tracings ' prepared by any of the 

 well-known methods, and which should immediately after- 

 wards be projected upon the screen, using the tracings as 

 ordinary scientific diagrams. To obtain such slides, either 

 the revolving blackened cylinder so usual hi tracing apparatus 

 must be displaced for tracing on the flat, which is most 



