198 Professor William H. Bragg [June 5, 



WEEKLY EVENING MEETING, 



Fjiday, June 5, 191-1:. 



Charles Hawksley, Esq., M.Insf.C.E., Vice-President, 

 in the Chair. 



Professor William H. Bragg, M.A. F.R.S. 



X-Rays and Crystalline Structure. 



Two years have gone by since Dr. Lane made his surprising dis- 

 <;overy of the interference effects accompanying the passage of X-rays 

 through crystals. The pioneer experiment has opened the way for 

 many others, and a very large amount of work, theoretical and 

 practical, has now been done. As the preliminary exploration of 

 the new country has proceeded, our first estimate of its resources 

 has grown continuously ; we have learned many things which help us 

 to a better understanding of phenomena already familiar, and we 

 have seen avenues of enquiry open out before us which as yet there 

 has been little time to follow. The work is full of opportunities 

 for exact quantitative measurement, where precision is sure to bring 

 its due reward. There is enough work in sight to absorb the ener- 

 iiies of many experimenters, and there is sure to be far more than 

 we can see. When we consider the wideness of the new field, the 

 quality and quantity of the work to be done in it, and the import- 

 i'uce of the issues, we are scarcely guilty of over-statement if we 

 -say that Lane's experiment has led to the development of a new science. 



The experiment itself — to put it very briefly— constitutes a proof 

 that X-rays consist of extremely short ether waves. In order to 

 appreciate the value of this demonstration, we must bear in mind 

 the present conditions of our knowledge of the laws of radiation in 

 general. Let us consider very shortly how the whole matter stood 

 when the new vrork was begun. 



When X-rays were first discovered eighteen years ago it was 

 soon pointed out that they might consist of electro-magnetic dis- 

 turbance of the ether analogous to '.hose supposed to constitute light. 

 It w^as true that the new rays seemed to be incapable of reflection, 

 refraction, diffraction, and interference, which were familiar optical 

 23henomena. But it was pointed out by Schuster* that these defects 

 could be explained as natural consequences of an extremely small 

 wave-length. The positive evidence consisted mainly in the know- 



* Nature, Jan. 23, 1896. 



