32 
NATURE 
[NoveMBER 12, 1896 
will appear later). 
thus :— 
(N) Gap between negative knob terminal of Wimshurst and 
coiled wire to kathode. 
(KX) Gap between coiled wire to kathode and the kathode 
external wire loop. 
(P) Gap between positive knob terminal of Wimshurst and 
coiled wire to anode. 
(A) Gap between coiled wire to anode and the anode’s 
external wire loop. 
Experiment i.—Make N and P about 4”, A exceedingly small, 
and K about +”. The result, when the machine is being turned 
by hand as quickly as is convenient, is a series of discharges, 
during each of which the tube flashes out a brilliant, almost orange- 
green, giving on my fluorescent screen a light so bright as to be 
trying to the eyes ; the hand will show at once that X,-rays are 
being emitted, and with this arrangement I have shown the 
ordinary shadow experiments to a large room-full at once. Three 
or four persons can see at the same time the back-bone and ribs of a 
man if a little care is taken to exclude extraneous light ; and what 
I expect will interest a great many people who are not scientific, 
I have seen (to put it popularly, though of course incorrectly) 
through a brick wall, ze. through 84 inches of solid brick. To 
speak exactly—the rays which come through the brick are suffi- 
ciently powerful to show dullish flashes on the screen, and a 
piece of platinum foil placed just behind the screen is distinctly 
visible, though badly defined, during the flashes. It is necessary, 
of course, to be careful to avoid any ordinary light reaching the 
screen, or any X-rays from reaching it except through the wall ; 
but the experiment has been performed carefully many times now, 
and there is no doubt whatever as to the power of the rays from 
even my small tubes to penetrate this thickness of brick. These 
X-rays have wonderful penetrating power, and experiments with 
them are well worth making. The X-light is able to penetrate 
a little over 13” of glass, and 3” of water easily, and 37” of wood. 
In this experiment, unless the spark gaps are adjusted carefully 
(and the measurements I give are only intended as a rough guide), 
sparks pass along the outside of the tube in a way which at first 
made me anxious for the tube’s life ; but so long as the tube’s loops 
are nearer to the coiled wires than any other part of the tube, 
experience teaches me there is no danger. Even during the 
passage of these external and noisy sparks, there is a discharge 
inside the tube sufficient to show a faint and very transparent 
hand shadow on the screen, the bones being scarcely distinguish- 
able from the flesh—these being the most transparent shadows 
I have yet seen. 
Experiment ii.—Arrange so that A is exceedingly small, 
and N and P about equal, and so that the discharge by long 
sparks along the tube and outside it are just avoided ; then take 
a piece of thoroughly wet string, and fasten one end of it to the 
kathode loop, and the other to the end of the coiled kathode 
wire (a few strands of lamp-wick answer even better), and move 
the kathode coiled wire away from the tube’s neighbourhood, 
carrying (of course) one end of the string with it until there is no 
direct sparking through the air, and all the discharge goes 
through the string. If the radiation from the tube be now 
examined by the screen with the hand held close behind it, the 
shadow of the flesh will be very dark indeed, and the bones 
scarcely visible ; in fact, it is fairly easy to secure a quite black 
shadow whilst the rest of the screen is brightly illuminated. 
These are, therefore, the X-rays. By shortening the string 
gradually the shadow changes, the flesh becoming more and 
more transparent, the bones’ shadows remaining black for some 
time, until when the direct disruptive discharge through the air 
begins the X,-rays are immediately restored, and the radiation 
penetrates easily both flesh and bone. Thus, by ‘‘ loading” the 
circuit more or less, the character of the radiation isaltered. We 
know that the discharge of a Leyden jar is oscillatory, and the 
frequency of vibration very high, also that the frequency is 
lowered and the oscillations damped by the use of a wet string 
For brevity I designate the spark gaps 
cr other resistance placed in the jar’s discharge circuit ; hence | 
it appears reasonable to suppose that it is this slackening of the 
electrical oscillations which produces the corresponding change | 
in the X-rays, and that this slackening can be produced by 
altering resistance outside as wellas inside the tube, at any point, 
perhaps, in the circuit, so long as the discharge is kept disruptive 
in character. In this experiment the discharge through the 
three spark gaps, and so through the whole circuit, is certainly 
disruptive ; but the influence of the wet string on the whole 
circuit, including, of course, these gaps, is shown strikingly by 
NO. f4NT, VOL sai) 
the altered appearance and sound of the sparks at N and P-. 
The wet string, therefore, causes the tube to produce rays of the 
same kind as the heating of the tube, and if heat increases the 
general conductivity, it would seem that the wet string must do 
the same thing. It is, besides, obvious that the wet string 
takes the place of a rather wide air gap, and it can be very 
easily proved by experiment that the air gap possesses the 
greater resistance. Hence it might be thought that the 
vibrations in the case of air would be more slowly executed ; 
but in these experiments we have to do, not with the ordinary 
resistances of air and wet string, but with their resistances during 
the disruptive discharge ; to put it into popular language, the air 
can stand a great electrical stress without giving way, but when 
once its initial and great resistance is overcome, its resistance 
may be for the time very greatly diminished, so that the electri- 
city surges backwards and forwards, it may be, many millions of 
times ina second ; whereas in the case of the wet string, although 
it opposes at first far less than.the air, yet its resistance never 
breaks down completely, for this very reason. The air and the 
string may be compared in this respect to an oak and a reed ina 
gale of wind; the reed, though it bends still resists, is always 
resisting, and the more it is bent the more it resists; the oak 
stands unmoved till it is broken or uprooted, and its resistance 
overcome. That the oscillatory discharge is necessary for the 
production of the X-rays I feel no doubt, though it would be an 
extremely interesting experiment for any one, who had the means 
and leisure, to try whether they could be produced by a so-called 
continuous current from a battery. A very few thousands of 
cells would seem likely to be necessary ; but if any one should 
construct a tube according to the hints and instructions given in 
this letter, I think that the number which would be found 
necessary might be a good deal less. The discharge of a 
Wimshurst without Leydens, and with the four spark gaps 
nil, is still disruptive, though I expect less so when (unlike mine) 
the plates have no metal buttons and sectors on them; the 
rotating mirror, I believe, shows that even the silent brush 
discharge is disruptive—certainly the thin blue sparks are so, and 
the discharges from the revolving plates on to the combs and 
between the two plates themselves, are all disruptive. 
Experiment iii.—The spark gaps being arranged as in Experi- 
ment ii., instead of the wet string the secondary circuit of my 
44” spark induction coil was inserted, the rays given off by the 
tube were then X,-rays, which gave most exquisitely bright and 
clear pictures on the screen, whilst at the same time the discharge 
through the coil gave rise to very decided Hertz effects for a 
considerable distance. It is clear that a rheostat of some fine 
wire of high resistance would be convenient to use with a tube, 
and that by its means we could adjust the kind of X-ray evolved 
very nicely. 
The nature of the spark discharges at N and P during the 
emission of the X-rays deserves close attention. At times the 
spark seems like a string of equidistant silver beads strung on a 
bluish violet thread, and suggests stationary waves. Perhaps I 
may add that my work is necessarily interrupted for some 
weeks at least. I hope that any one who finds in this letter 
suggestions he would like to follow up experimentally, will not 
fail to carry out his wishes. The form of kathode I should 
recommend is a concave mirror focused on the anode, the outer 
rim being bent back so that the edge is well concealed behind 
the mirror, a section through the centre of the mirror face 
having a form something like a very shallow sign of Aries. 
Eton College, July 24. T. C. PORTER. 
Extension of the Visible Spectrum. 
REFERRING to the interesting letter on the above subject, 
from Prof. Oliver Lodge and Mr. B. Davies, in your issue for the 
20th ult., I should like to mention that I have observed a similar 
extension of the visible spectrum when thrown upon a fluorescent 
screen of barium platino-cyanide. I may add that a screen of 
this description becomes brilliantly luminescent when brought 
into the vicinity of the brush discharge from a large Wimshurst 
machine, or, better still, froma Tesla coil. In the latter case the 
fluorescent surface will become luminescent at a considerable 
distance from the electrical discharge, if facing the latter ; but if 
the screen is held with its opaque backing towards the discharge, 
the platino-cyanide will only luminesce if actually penetrated by 
the streamers of the discharge. 
This screen will also fluoresce brightly at several yards 
distance from an ordinary Geissler vacuum tube. 
