Sensitive ae ss of the Ear- to Pitcli. 



601 



easily audible, but sometimes a little more was allowed. 

 The amplitude of vibration at a loop L was then measured by 



Fiff. 1. 



means o£ a microscope provided with a micrometer-scale and 

 focussed upon starch grains carried by the rim. In passing 

 to the can under comparison the distance of observation is no 

 longer variable at pleasure but must be taken in proportion 

 to the linear dimension. Thus if the second can be on half 

 the scale of the first and sound the octave above, the distance 

 must be halved. If, when listened to in the four positions, 

 the sound is judged too strong or too faint, the vibration must 

 be modified (by varying interposed resistance) until the 

 former audibility is reproduced. The amplitude of vibration 

 at a loop is then measured with a second microscope similar 

 to the first. The distances of the ear, measured from the rim, 

 varied from 8 inches to 21 inches. The sounds under com- 

 parison were usually estimated independently by Mr. Enock 

 and myself. A slight tendency on my part to estimate the 

 graver sounds as the louder was suspected, but the difference 

 was of no importance. Other observers also have taken part 

 occasionally, and there was sufficient repetition on different 

 days to eliminate chance errors. It will suffice to record the 

 mean results. 



In the comparison of cans of dimensions in the ratio of 2 : 1 

 and making 128 and 256 vibrations per second, it was found 



