184 



Mr. G. Pierce on Indices of 



In taking the observations, in order to eliminate the 

 irregularities due to fluctuations in the source of the waves, 

 the readings are made comparative in the following manner : 



A point on the scale is chosen as standard for a given set 

 of readings, and the deflexions when the reflector is at any 



Fkr. 5. 



required point are compared by alternate readings with the 

 deflexions when the reflector is at the standard point. The 

 average of those for the required point divided by the average 

 for the standard point, gives a ratio that represents the re- 

 sultant intensity for the required point. This ratio can be 

 veriried at any time, and is independent of the order of taking- 

 observations and of small random variations of the spark. 



For example, in determining a stationary wave in air, the 

 point marked 13 on the scale was conveniently chosen as the 

 standard point. The intensity with the reflector at any other 

 point, say the point marked 8*3 on the scale, was compared 



