796 



SCIENCE 



[N. S. Vol. XL. No. 1040 



constitute light. It was true that the new rays 

 seemed to be incapable of reflection, refraction, 

 diffraction and interference, which were famil- 

 iar optical phenomena. But it was pointed 

 out by Schuster^ that these defects could be 

 explained as natural consequences of an ex- 

 tremely small wave-length. The positive evi- 

 dence consisted mainly in the knowledge that 

 the impact of the electrons on the anti-cathode 

 of the X-ray bulb ought to be the occasion of 

 electro-magnetic waves of some sort, and in 

 the discovery by Barkla that the X-rays could 

 be polarized, which last is a property also of 

 light. 



As experimental evidence accumulated, a 

 number of results were found which the electro- 

 magnetic theory was unable to explain, at least 

 in a direct and simple manner. They were 

 mainly concerned with the transference of 

 energy from place to place. In some way or 

 other the swiftly moving electron of the X-ray 

 bulb transfers its energy to the X-ray, and the 

 X-ray in its turn communicates approximately 

 the same quantity of energy to the electron 

 which originates from matter lying in the 

 track of the X-ray, and which is apparently 

 the direct cause of all X-ray effects. Experi- 

 ment seemed to indicate that X-ray energy 

 traveled as a stream of separate entities or 

 quanta, the energy of the quantum differing 

 according to the quality of the X-ray. It 

 looked at one time as if it might be the sim- 

 plest plan to deny the identity in nature of 

 X-rays and light, to describe the former as a 

 corpuscular radiation and the latter as a wave 

 motion. Otherwise, it seemed that the electro- 

 magnetic hypothesis would be torn to pieces 

 in the effort to hold all the facts together. 



But it appeared on a close examination of 

 light phenomena also, though in much less 

 obvious fashion, that the very same effects 

 occurred which in the case of X-rays were so 

 difficult to explain from an orthodox point of 

 view. In the end it became less difficult to 

 deny the completeness of the orthodox theory 

 than the identity in nature of light and X-rays. 

 Modern work on the distribution of energy in 

 the spectrum, and the dependence of specific 



2 Nature, January 23, 1896. 



heat upon temperature, has also led indepen- 

 dently to the same point of view. It has been 

 urged with great force by Planck, Einstein 

 and others that radiated energy is actually 

 transferred in definite units or quanta, and 

 not continuously; as if we had to conceive of 

 atoms of energy as well as of atoms of matter. 

 Let it be admitted at once that the quantum 

 theory and the orthodox theory appear to 

 stand in irreconcilable opposition. Each by 

 itself correlates great series of facts; but they 

 do not correlate the same series. In some way 

 or other the greater theory must be found, of 

 which each is a partial expression. 



The new discovery does not solve our diffi- 

 culty at once, but it does two very important 

 things. In the first place, it shows that the 

 X-rays and light are identical in nature; in 

 fact, it removes every difference except in re- 

 spect to wave-length. The question as to the 

 exact place where the difficulty lies is decided 

 for us; we are set the task of discovering how 

 a continuous wave motion, in a continuous 

 medium, can be reconciled with discontinu- 

 ous transferences of radiation energy. Some 

 solution there must be to this problem. The 

 second important thing is that the new meth- 

 ods will surely help us on the way to find that 

 solution. We can now examine the X-rays as 

 critically as we have been able to study light, 

 by means of the spectrometer. The wave- 

 length of the X-ray has emerged as a measura- 

 ble quantity. The complete range of electro- 

 magnetic radiations now lies before us. At 

 one end are the long waves of wireless teleg- 

 raphy, in the middle are first the waves of the 

 infra-red detected by their heating effects, then 

 the light waves, and then the short waves of 

 the ultra-violet. At the other end are the 

 extremely short waves that belong to X-radia- 

 tion. In the comparative study of the prop- 

 erties of radiation over this very wide range 

 we must surely find the answer to the greatest 

 question of modern physics. 



So much for the general question. Let us 

 now consider the procedure of the new inves- 

 tigations, and afterwards one or two applica- 

 tions to special lines of enquiry. 



The experiment due to Laue and his coUab- 



