PHYSICS: A. G. WEBSTER 
179 
harmonically varying currents. I accordingly adopted it instead of the inter- 
rupting tuning fork, inasmuch as it could be instantaneously tuned to any 
pitch whatever, and we are therefore freed from the necessity of being confined 
to four or five pitches, but have perfectly continuous range. The intensity 
of the sound emitted is measured by the amplitude of [vibration of the disk, 
read off by the micrometer 10, focused on a fine slit ruled in the silvering of a 
piece of cover-glass 5 carried by the disk, and illuminated by an incandescent 
lamp and condenser not shown. It may be remarked that this phone as it 
stands may be used as a phonometer for tolerably loud sounds, such as those 
of singing near by, which gives a considerable vibration in the microscope. 
It should be likewise stated that this phone produces sound more efficiently 
than any other known instrument. 
The theory of the phonometer and the phone is given in another paper. 
