28 PHYSICAL RESEARCH 



purely a fancy picture ; no such powers were given, 

 at the time he wrote the book, to any being; and 

 we are no nearer the realisation of this dream than 

 we were then. The things that are coming, the 

 powers we shall possess in the future, and the 

 insight we shall gain, will be very wonderful; we 

 may be quite sure of that. But we cannot really 

 anticipate, and when we try to prophesy and to 

 imagine we are all wrong. Let us take an illustra- 

 tion from actual experience, and try to imagine 

 the development of a certain invention in the 

 reverse fashion; the fashion which is so often 

 supposed to be the true, and which is absolutely 

 false. 



We all know now some of the things which 

 X-rays can do. We know that they permit us 

 to have knowledge of the interior of bodies opaque 

 to ordinary light. In particular the surgeon and 

 physician can now examine the interior of the 

 human body, and they find in the X-rays one of 

 the most valuable tools ever placed within their 

 power. Let us consider the manner of their 

 discovery, and in order to appreciate that, let us 

 first consider very briefly their nature and properties. 

 X-rays are really rays of light, except that they 

 have wave lengths ten thousand times shorter than 

 light which is seen by the eye. They are produced 

 inside a glass bulb, highly evacuated of air, by 

 driving an electric discharge through the bulb. 

 The driving is done by an induction coil, which 



