moves the faster moves the image of the light. In this way a speed of a millionth of a second can be attained. In this case the distance between the dots on the film may be one tenth of an inch, sufficient to separate them to the eye. The photograph of electric sparks (rig. 8) was taken in this manner. The distance between any two bright spots in the trail of the photographic images represents the time of the electric oscillation or the time of
the magnetic pulse or wave which is sent out from the spark, and which will cause a distant circuit to respond by a similar oscillation.
At present the shortest time that can, so to speak, be photographed in this manner is about one two-millionth of a second. This is the time of propagation of a magnetic wave over four hundred feet long. The waves used in wireless telegraphy are not more than four feet in length—about one hundredth the length of those we can photograph. The photographic method thus reveals a mechanism of the spark which is entirely hidden from the eye and will always be concealed from human sight. It reveals, however, a greater mystery which it seems incompetent to solve—the mystery of what is called the pilot spark, the first discharge which we see on our photograph (Fig. 9) stretching intact from terminal to terminal, having the prodigious velocity of one hundred and eighty thousand miles a second. None of our experimental devices suffice to penetrate the mystery of this discharge. It is this pilot spark which is chiefly instrumental in sending out the magnetic pulses or waves which are powerful enough to reach forty or fifty miles. The preponderating influence of this pilot spark—so called since it finds a way for the subsequent surgings