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wave-lengths if Röntgen-rays are simple periodical disturbances : 
for plate A, B: en 
A= 016 0,05 Os 12E is 
Now that this supposition does certainly not hold, we shall have 
to consider these values as estimations of wave-lengths, which in 
the three different experiments have been more or less prominent 
in the curve of energy *) of the Röntgen-rays. 
Mention ought to be made here, that, although not too much im- 
portance must be attached to the three values of 4 as far as the 
absolute figures go, the difference they show is probably real and 
connected with the difference in hardness of the tubes. As was 
mentioned above the tubes used for plate were distinguished 
by a considerable hardness from the others, which were relatively 
very soft. 
Worth noticing is also the fact, that the values of À found here 
are of the same order as those deduced from our former experiments. 
Finally we wish to state emphatically that we continue to regard 
as the chief result of our investigations the proof they furnish that 
the Röntgen-rays ought to be considered as a phenomenon of radiation 
in the ether. 
Physical Laboratory University Groningen. 
Physics. — H. A. Lorentz. “The fundamental equations for electro- 
magnetic phenomena in ponderable bodies, deduced from the 
theory of electrons.” 
$ 1. In framing a theory that seeks to explain all electromagnetic 
phenomena, in so far as they do not take place in free aether, by 
means of small charged particles, electrons, we have to start from 
two kinds of equations, one relating to the changes of state in the 
aether, the other determining the forces exerted by this medium on 
the electrons. To these formulae we have to add properly chosen 
assumptions concerning the electrons existing in dielectrics, conductors 
and magnetizable substances, and the forees with which the ponder- 
able particles act on the electrons in these several cases. 
In former applications of the theory I have restricted myself to 
1) GC. H. Wino. Il. ec, 
