AUGUST 25, 1899.] 
invisible, is in reality easily visible if of 
great intensity. The parts of the retina 
which respond and so give the sensation of 
luminosity are apparently those around the 
eye and not directly opposite to the iris 
opening. Those parts of the retina sensi- 
tive to the rays are characterized by the 
preponderance of ‘rods,’ giving the sim- 
ple sensation of illumination, apparently 
white in the case in question. The ‘cones,’ 
or those portions of the retinal membrane 
whose function is believed to be the recog- 
nition of color or differences of wave- 
length, appear not to be excited by the 
Rontgen radiation, or only very feebly. 
If this be true it would account for the less 
intensity of the luminous effect upon those 
portions of the retina near the optic axis of 
the eye. All this favors the view that the 
Rontgen radiation is without sustained 
pitch or wave trains, and resembles more a 
sharp noise or crash in sound. 
For pressing experimental work in the 
highest vacua to its limit, as above sug- 
gested, we already have means at command 
for the production of the most complete ex- 
haustions, requiring extremely high poten- 
tials to pass an electric discharge. We 
have, also, in well-known forms of high-fre- 
quency apparatus the means for producing 
electromotive forces limited only by our 
means for insulation. A recent apparatus 
devised by me and called a dynamostatic 
machine gives equal capability of produc- 
ing high potentials of definite polarity, 
positive and negative. It should not he 
long, therefore, before work is undertaken 
in this suggested direction of pressing this 
matter of rays of high penetrating power 
much farther than has been done. The 
question arises whether any such rays can 
exist which are not appreciably absorbed in 
passing through dense substances. They 
would probably not affect a photographic 
plate nor a fluorescent screen. If they lost 
also the property of ionizing a gas and 
SCIENCE. 241 
causing electric convection we might not 
even be able to discover them. That some 
influence or action in the ether does ac- 
tually penetrate the dense masses in space 
is evidenced by gravitation, the mystery of 
mysteries. We are, however, not justified 
in going beyond the proved facts which can 
only be the result of experimental work 
and close observation. All else is specula- 
tion. The energy source of the Becquerel 
rays is another mystery apparently far 
from being cleared up, and if it be true, 
as recently announced, that a substance 
named radium has in reality nine hundred 
times the power of emitting these rays than 
is possessed by uranium and thorium, and 
that the radiation is able to cause visible 
fluorescence of barium platinocyanide, the 
mystery but deepens and makes us again 
think of the possible existence of obscure 
rays only absorbed and converted by a few 
special substances. 
The diffusion which takes place when 
Rontgen rays pass through various media 
is another phenomenon which needs more 
attention from investigators. This effect 
seems to be produced by all substances in a 
greater or less degree. It, however, appears 
to be nearly absent in the case of those sub- 
stances which give out light or fluoresce 
under the rays, as barium platinocyanide 
and calcium tungstate. It willbe important 
to determine definitely whether the rays dif- 
fused by different substances are lowered in 
pitch or penetrating power as compared with 
the rays exciting the diffusion ; whether, in 
other words, the rays from a tube with quite 
high vacuum excite similar rays by diffu- 
sion, or rays more absorbable; and if a 
lowering takes place whether it occurs in 
like manner and degree for all diffusing 
media. 
The phenomenon may be akin to fluores- 
cence, as when quinia sulphate converts the 
invisible ultraviolet rays of the spectrum 
into lower rays or visible light. This 
