22 



EXI'EIM.M KNTAL PHYSIOLOGY 



to record the amount of contraction of a frog's muscle we may fix 

 one end to some rigid support and to the free end attach some form 

 of writing-point, which is made to record its movement upon a piece 

 of paper so placed that the point, during its movement, is always in 

 contact with the paper. "Where the amount of movement to be 

 recorded is small, it is readily magnified by some form of lever such 

 as one of those represented in figs. 31 and 37. We in this way obtain 

 a straight or curved line which gives us at once a permanent record 

 of the amount of movement performed, or of some multiple of it. We 

 still have one important point to determine in the consideration of any 

 movement, viz. the time occupied. Thomas Young was the first to 

 show how we might obtain measurements of time with very consider- 

 able accuracy. He pointed out that if a surface be moved in a given 

 direction at a constant rate, lines measured parallel to the direction 

 of motion indicated time, and that to determine the value of those 

 lines all that was necessary was to fix a very light style or marker to 

 a vibrating rod, held so that the style was in contact with the moving 

 surface and its movements at right angles to the direction of motion 

 of the surface. If the time of oscillation of the rod be known, the rate 

 of movement of the surface is directly determined. This time measure- 

 ment was perfected by Duhamel by employing a tuning-fork to one 

 prong of which a light writing-point is fixed. The rate of vibration 

 of the tuning-fork can be determined with very great accuracy, and 

 hence the rate of movement of the surface can be determined with the 

 same accuracy. The recording surface, which is most convenient and 

 which is usually employed, consists of a smooth and highly glazed 

 surface of paper, which is covered with a thin deposit of carbon, 

 obtained by holding it in a smoky flame of burning gas, camphor, 

 turpentine, or some other substance. The writing-point is made of 

 metal, glass, or moderately stiff paper cut to a sharp point, which is 



Fig. 27. — Two Records of the Vibrations of a Tuning-fokk Vibrating at 

 the Rate of 10 per sec. The Tracing a b was taken while the Record- 

 ing Surface was Moving more Rapidly than during the Record c d. 



then made to scratch the smoked surface, and so remove some of the 

 black deposit and bring the white surface of the paper into view. The 



