476 KANSAS CITY REVIEW OF SCIENCE. 
end of a conducting wire so nearly that when at rest the interval between the" 
two points can be discovered only through a strong lens. The thing to be done 
was to show in a picture of the instrument, or rather in a series of pictures, the 
alternate contact and separation of the points from the vibrations imparted to the 
diaphragm by the voice, involving the closing and opening of the electrical cir- 
cuit and the consequent reproduction of the same rate of vibration in the receiv- 
ing instrument at the other end of the line. In considering this problem Mr.. 
Rockwood found himself indebted to his recollection of an experiment by Her- 
schel in photographing (or daguerreotyping) with the electric spark. Herschel 
caused a four-sided prism of wood, around which a picture was pasted, to revolve 
at high rate of speed as in a turning lathe. By illuminating this revolving pic- 
ture with the electric spark (in total darkness otherwise) he obtained a photograph' 
of it as standing still at that instant in its revolution when the spark flashed. Im 
describing his experiment Mr. Rockwood said : 
" Wheatstone measured the duration of the electric spark as one-twenty- 
four thousandth of a second. It would follow that any vibration not quicker than- 
this might be arrested on the photographic plate at any point in its travel. 
Whereas, according to the investigations of Plateau, the duration of successive- 
impressions on the human eye will average half a second, the electric spark might 
separate and distinguish photographically waves of which 12 ooo impinge on the 
retina while the first of them is still lingering there; in other words, 12,000 prac- 
tically all at once. Now the vibrations or waves of air that yield the respective 
tones or pitches of sound have been accurately measured and counted. Assum- 
ing the pitch of the ordinary masculine voice in conversation to be as low aS' 
middle C, the number of complete double vibrations imparted by such a voice tO' 
the telephonic diaphragm or tympanum would be 256 per second; that is, count- 
ing both ways, 512 movements of the diaphragm v/ith its metallic point making 
or breaking the electrical circuit at each movement. To the eye, which retains 
every impression for half a second, 256 of these movements would make their 
impression as one, and would give a stationary, fictitious image, if they were of 
sufficient depth to produce any visible effect whatever, as they are not. The- 
electric spark, however, would give its illumination and do its photographic 
work within a little more than one-fiftieth of the time of one of the tympanum 
movements. A succession of such photographs, therefore, would present fortuit- 
ously any position of the vibrating point from that of contact to that of extreme 
retraction, with an indefinite number of intermediate positions." 
To verify these calculations, Mr. Rockwood carefully focused his photo- 
graphic camera on the points of the telephone by daylight, and a battery of 
Leyden jars was so adjusted that when discharged it would throw the proper 
illumination on the points. Mr. Rockwood's instantaneous plates were now to 
be tested under action five hundred times quicker than a sensible instant and also^ 
invisibly minute. Of course it was as yet a practical question whether they could 
effectively receive as quickly as the electric spark would give this infinitesimal 
action of light. Waiting until the darkest hour of the night, the plate was un- 
