124 THE FIELD OF EXPERIMENTAL RESEARCH. 



but may be an unavoidable incident of the very sudden discharge which 

 itself would set up a single pulse in the ether sufficiently intense for 

 the work even if unaccompanied by lower amplitude oscillations fol- 

 f owing the first discharge pulse. 



Before leaving the consideration of this most fruitful field of 

 experimental research opened by Hertz it may be stated that the one 

 gap in the work yet to be filled is the actual production of electric 

 waves of a wave length corresponding to those of the spectrum. If 

 this could be done by some direct method, no matter how feeble the 

 eflect obtained, the experimental demonstrations of the electric nature 

 of radiant heat and light would be fith'^ completed. Several years ago 

 it occurred to me that it might be possil)le to devise a method for 

 accomplishing the end in view, and so close the existing gap. Many 

 years ago an observation on sound echoes showed clearly the produc- 

 tion of high-pitch sounds from single pulses, or lower-pitch waves. 

 A bridge over a mile in length was boardt'd at the sides, and vertical 

 slats regularly and closely placed along its side formed, for a sound 

 wave incident thereon, a series of reflecting edges or narrow vertical 

 surfaces, a kind of coarse grating. It was found that a loud sound or 

 ])ulse. such as that of a gunshot, emanating from a point near one end 

 of the bridge and two to three hundred feet in a line from the struc- 

 ture, was followed by an echo, Avhich was in reality a high-pitch musical 

 tone. The pitch of this tone corresponded to the spacing of the slats 

 in the ))i-i(lgc considered as a reflecting grating for sound. 



Following this principle, it seems possible that a very sudden pulse 

 in the ether or electromagnetic wave, incident at an angle upon a 

 reflecting grating liaving from 20,000 to -10,000 ruled lines to the inch, 

 if the plane of incidents were at right angles with the rulings, might 

 be thrown into ripples of the wave length of light and yield a feeble 

 luminosity. If the color then varied with the angle of incidence chosen 

 and with the angle through which the reflection passed to the eye the 

 experiment would be conclusive. 



Despite the diligent studies which had been made in the invisible 

 rays of the spectrum, both the ultrared and ultraviolet, a work far 

 from completion as j^et, the peculiar invisible radiation of the Crookes 

 tube remained unknown until the work of Lenard and Kontgen 

 brought it to the knowledge of the world. The cathode discharge, 

 studied so efl'ectivelj'' by Hittorf and Crookes, and by the latter called 

 "radiant matter," was but a part of the whole truth in relation to the 

 radiation in high vacua. It is needless to recount the steps in the dis- 

 covery of Rontgeu rays. We now know that these rays come from 

 the impingement of the " radiant matter," or cathode rays. We know, 

 also, that the higher the vacuum, and therefore the higher the elec- 

 tric potential needed to effect the discharge, the more peneti'ating 

 or the less easily absorbed is the resulting radiation. Rays have 



