FEBRUARY II, 1897 } 
I have nowhere called the undulatory ¢heory of light an 
hypothesis ; I have called the undulatory conceptzon (Vorstellung) 
of light an hypothesis. I have nowhere said that ‘‘it is the 
sinniiche Wahrnehmung which changes the Aypothese to the 
Naturgesets.” Perception, as was seen in the case of photo- 
graphs of light-waves obtained by Prof. Wiener, makes 
hypothesis a matter of fact. To call this fact a law, has never 
come into my mind. 
I have nowhere stated Galilei’s law of inertia to be a law of 
nature ; I have declared it to be a postulate laid down on 
the basis of rich empirical materials (ein auf grundreichen 
empirischen Materials aufgestelltes Postulat, p. 169). Newton’s 
law of gravitation and the principle of energy are typical 
examples of natural laws. The law of inertia belongs to the 
“*axiomata sive leges motus” of Newton, and in this respect I 
have compared the law of inertia to the axioms of geometry, 
and, considering the rich experience of ¢o-day, I have spoken as 
of an appearance of obviousness and immediateness (von einem 
Scheine des Einleuchtenden und Unmittelbaren,p. 168). Howlittle 
I really think the law of inertia to be so obvious and immediate 
(so einleuchtend und so unmittelbar), my reviewer might also 
have learned from pages 74-76 of my book. In a postulate, as 
well as in a law of nature, immediate perception is entirely out 
of the question. 
These few examples will suffice to show that my reviewer, 
perhaps owing to some difficulties in understanding the German 
language, has constructed certain ideas which, in fact, do not 
exist at all, and are, by no means, advocated by myself. As it 
is, I do not feel obliged to enter into a discussion of his other 
remarks, in which he, at least, quotes from my book correctly. 
PAUL VOLKMANN. 
University of Kénigsberg, January 18. 
For our Principle of Inertia the Germans use two expressions, 
Traghettsprincip and Tragheztsgesetz. Dr. Volkmann in his 
work almost invariably adopts the latter, and even speaks of the 
principle of inertia as das phystkalische Gesetz der Traghett. If 
a physical law be not a law of nature, Dr. Volkmann ought to 
have carefully distinguished between a Naturgesefs and a 
phystkalisches Gesetz. Iam sorry, however, to have given him 
more credit for logical consistency than he desires to lay claim 
to. The law of inertia describes how an insensible particle 
would move relatively to certain ‘‘ fixed axes” under certain 
purely conceptual, not physically realisable conditions. The law 
of gravitation describes how two insensible particles would move 
relatively to certain ‘‘fixed axes” under certain purely concep- 
tual, not physically realisable conditions. If Dr. Volkmann 
wishes to draw a distinction between the two, and calls the latter 
alone a law of nature, then my opinion of his Erkexntnzstheo- 
vetische Grundzuge is now lower than it was when I wrote my 
notice of it. Dr. Volkmann refers me to pp. 74-76 of his work 
to illustrate that he did not consider the law of inertia so 
einleuchtend und so unmittelbar, yet those are precisely the pages 
from which I cited in my review the motion of the railway train, 
which Dr. Volkmann considers can illustrate the law of inertia, 
an example which I hold to be most illusory. It is especially 
dangerous to the young student, and most misleading in popular 
lectures like Dr. Volkmann’s <Ad/gemetn wissenschaftliche 
Vortriage. 
Dr. Volkmann tells us that Wiener’s researches have changed 
the undulatory conception ( Vorste//ung) of light into ezme vollen- 
dete Thatsache, it has now ceased to be an hypothesis. How a 
conception can become a physical fact by any amount of research, 
I fail to understand ; although I do grasp how the contents of 
that conception may by new discoveries be found to adequately 
describe our physical sensations. But when the contents of the 
conception are found to adequately describe our sensations, then 
it seems to me that the orderly account of those contents, which 
we term theory, ceases to be hypothesis, and becomes a law of 
nature. If Wiener’s work makes the undulatory conception an 
adequate account of sensation, then it converts the undulatory 
theory into a law of nature. But Dr. Volkmann tells us this is 
not what he has said. Perhaps the undulatory theory, now that 
the undulatory conception has become ez7e vollendete Thatsache, 
still remains a theory, or is a postulate, or a physical law, or 
something else in Dr. Volkmann’s classification. I certainly do 
not sufficiently grasp Dr. Volkmann’s Grumdziige to be able to 
classify it. 
Lastly, I hope my ‘‘ understanding of the German language” 
NO. 1424, VOL. 55] 
NATURE 
343 
has misled me; otherwise I should say that Dr. Volkmann’s 
present interpretation of what he has written on p. 168 ‘‘appears” 
to me disingenuous. What he has written there runs :— 
““Wir befinden uns dem Tragheitsgesetz gegeniiber heute 
vielleicht in ahnlicher Lage wie der Geometer seinen Axiomen 
gegeniiber, dem es gerade darum so schwer fallt, an seinen 
elementaren Satzen erkenntnistheoretische Studien anzustellen, 
weil der Inhalt dieser Satze so einleuchtend, so unmittelbar 
zuganglich ist. So scheint dem Physiker heute das Tragheits- 
gesetz so einleuchtend, so unmittelbar, dass es als Axiom 
vorgetragen zu werden pflegt. Aber es gab eine Zeit, wo der 
Inhalt des Tragheitsgesetzes dem menschlichen Geiste durchaus 
nicht so unmittelbar zuganglich erschien, und dies werden wir 
uns zu vergegenwartigen haben, um die Bedeutung der Galilei- 
schen Forschung noch heute wiirdigen zu konnen,” 
I take this to mean that the law of inertia appears obvious to 
the physicist of to-day, but at the time of its discovery it was 
not at all an obvious conception. Dr. Volkmann says that in 
this passage he has spoken von ezrem Schetne des Einleuchten- 
den und Unmittelbaren. He has certainly used the verb schezner, 
but when I say, for example, that twice two makes four appears 
to me a direct and obvious truth, I certainly do not mean to in- 
dicate that the directness and obviousness are sfeczows. I must 
apologise to Dr. Volkmann for having misunderstood this 
subtlety of the German language. 
It is perhaps necessary to add that Dr. Volkmann is raising a 
verbal controversy which has nothing to do with our radical 
difference of view. For me a law of nature is purely a product 
of the human intellect ; it is a formula which describes in the 
briefest terms yet discovered as wide a range as possible of the 
motions we attribute to atoms, particles, molecules, ether, &c., 
which kinetic concepts form parts of the entirely conceptual 
model by aid of which we describe the sequences of our physical 
sensations. For Dr. Volkmann the law of nature is something 
existing ausser uns (p. 56) in some manner kept in harmony 
with the Denknothwendigketten in uns. Presumably the law lies 
in the Dinge an stich, for it would be impossible to find a law 
like that of gravitation in the contents of our physical sensations. 
It is at this point that the older view of the physical sciences, as 
something quite different from descréft2ve sciences, runs us 
aground on the metaphysical mudbank. KARL PEARSON. 
Durham Degrees in Science. 
Just two remarks in answer to the Rev. Henry Palin 
Gurney’s letter in your last issue. 
He admits by his silence my main contention, viz.—that the 
nature of the M.Sc. degree at Durham has been radically 
changed owing to the recent action of the University in granting 
the degree by vote of Convocation. 
I did not insinuate in my letter that ‘‘ these gentlemen had no 
qualification for the honour.” , 
It is impossible for me to know the necessary qualifications 
for the degree other than those published by the University in 
their Calendar. It was sufficient for me that the latter were 
ignored. xX. 
February 6. 
ON THE CONDUCTIVE EFFECT PRODUCED 
IN AIR BY RONTGEN RAYS AND BY 
ULTRA-VIOLET LIGHT. 
ye propose in this communication to describe results 
of experiments on the electrical effects of Rontgen 
rays and of ultra-violet light when shone on metals, or 
through air between two metals mutually insulated ; and 
electrified to begin with, by previously producing a 
difference of potentials between platinum electrodes of 
an electrometer metallically connected with them. In 
some of our experiments this potential-difference was 
zero, and the initial + electrifications of the opposed 
surfaces depended solely on difference of volta-electric 
quality between their opposed surfaces. 
To investigate the effects of Rontgen rays, a hollow 
cylinder of unpolished aluminium connected to the 
1 A paper by Lord Kelvin, Dr. J. C. Beattie, and Dr. Smoluchowski de 
Smolan, read before the Royal Society of Edinburgh, February 1. 
