240 SCIENCE. 
may be an unavoidable incident of the very 
sudden discharge which itself would set up 
a single pulse in the ether sufficiently in- 
tense for the work even if unaccompanied 
by lower amplitude oscillations following 
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 
effect obtained, the experimental demonstra- 
tions of the electric nature of radiant heat 
and light would be fitly completed. Several 
years ago it occurred to me that it might be 
possible to devise a method for accom- 
plishing the end in view, and so close the 
existing gap. Many years ago an observa- 
tion on sound echoes showed clearly the 
production of high-pitch sounds from 
single pulses, or lower-pitch waves. A 
bridge over a mile in length was boarded 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 pulse, such as that of 
a gun-shot, emanating from a point near 
one end of the bridge and two to three hun- 
dred feet in a line from the structure, was 
followed by an echo which was in reality a 
high-pitch musical tone. The pitch of 
this tone corresponded to the spacing of the 
slats in'the bridge considered asa 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 having from 20,- 
000 to 40,000 ruled lines to the inch, if the 
plane of incidents were at right angles with 
the rulings, might be thrown into ripples of 
[N. S. Von. X. No. 243. 
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 con- 
clusive. 
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 yet, the pe- 
culiar invisible radiation of the Crookes 
tube remained unknown until the work of 
Lenard and Rontgen brought it to the 
knowledge of the world. The cathode dis- 
charge, studied so effectively by Hittorf 
and Crookes, and by the latter called ‘ ra- 
diant 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 discovery of Rontgen rays. We 
now know that these rays come from the 
impingement of the ‘radiant matter’ or ca- 
thode rays. We know, also, that the higher 
the vacuum, and, therefore, the higher the 
electric potential needed to effect the dis- 
charge, the more penetrating or the less 
easily absorbed is the resulting radiation. - 
Rays have been produced which in part 
pass through cast iron nearly an inch thick. 
The iron acting as a filter absorbs all rays 
of less penetrating power. <A question may 
here be put, which it will be for future ex- 
periment to answer: Can we, by increas- 
ing the degree of vacuum in a Crookes 
tube by the employment of enormous 
potentials for forcing a discharge through 
the higher vacuum, produce rays of greater 
and greater penetrating power? What, in 
fact, may be the limit, or is there any limit, 
to the diminution of wave-length in the 
ether, assuming for the moment that this 
invisible radiation is somewhat of the same 
nature as light, but of higher pitch, though 
it may be unlike light in not representing 
regular wave trains. 
Rontgen radiation, while spoken of as 
