242 LECTURES TO SCIENCE TEACHERS. 



the wrist and forearm. Attached to the frame is a spring, 

 which rests on and presses the artery. This being the case, 

 the spring fallows exactly the movements of the artery. 

 The rest of the instrument consists essentially of a long 

 lever by which the motion of the spring i.e. of the artery 

 is written on a plate which is made to travel at a uniform 

 rate by watch- work. 



This is one application of the direct method. There are 

 several other examples which might be taken, but I must 

 hasten on to describe to you the method of mediate trans- 

 mission. 



Here is an apparatus of M. Marey's which is intended 

 to illustrate this. It consists of two tympana, constructed 

 as I have been describing to you, and in communication 

 with each other by a flexible tube of several yards in 

 length ; notwithstanding this, any motion which I com- 

 municate to the near, or, as we may call it, the receiving 

 tympanum, is as perfectly transmitted to the distant or 

 recording tympanum as if the two membranes were 

 stretched over the end of one cylinder. 



It is obvious to you that the value of this contrivance, 

 as a means of investigating physiological motions, depends 

 entirely on its accuracy. It is therefore of fundamental 

 importance to be certain that the motions communicated 

 are faithfully imitated. 



In the beautiful collection of instruments exhibited by 

 Prof. Donders there is one (No. 3,955) which is specially 

 designed for this purpose. It was designated in the first 

 edition of the Catalogue by the rather quaint name of 

 " Controller of the Air Conveyance." It consists of three 

 essential parts, namely first, a cam, 1 by the rotation of 

 which a very complicated series of up-and-down motions 

 are communicated to any body capable of such motions, 

 which reposes on its upper <edge ; secondly, a lever for 

 direct transmission of these motions to a recording surface, 

 of the same kind as those already described to you ; and 

 thirdly, a couple of tympana of which the cavities are in 



1 The cam used in this experiment was of such form that the suc- 

 cessive variations of radius in each revolution corresponded very closely 

 to those of the transverse diameter of the heart of an animal ; so that 

 the curves or tracings recorded closely resembled those which are ob- 

 tained when the motions of that organ are recorded by the cardiograph. 



