294 BF-LL SYSTEM TECHXIC.IL JOCRX.IL 



with energies amounting to tens of tiioiisands of c(iiii\'aienl \'olts, cio 

 possess it. Tiiis is in fact liie process of excitation of X-ra\s, wliich 

 are radiated from a metal target exposed to an intense bombardment 

 of fast electrons. The protagonists of the electromagnetic theor>^ 

 had an explanation ready for this effect, as soon as it was discovered. 

 A fast electron, colliding with a metal plate, is brought to rest by a 

 slowing-down process, which might be gradual or abrupt, uniform or 

 satcade, but in any case must be continuous. Slowing-down entails 

 radiation; the radiation is not oscillator^', for the electron is not 

 (jscillating, but it is radiation none the less; it is an outward-spreading 

 single pulsation or pulse, comparable to the narrow spherical shell of 

 condensed air whidi diverges outward through the atmosphere from 

 an electric spark and has been photographed so often, or to a transient 

 in an electrical circuit. 



One may object lh.it the pulse is just a i)ulse and nothinj; niori'. w liile 

 the X-rays are wa\e-trains, for otherwise the X-ra\' spectroscope 

 (which is a diffraction apparatus) would not function. The objection 

 is answered by pointing out the quite indubitable fact that any pulse, 

 whatever its shape (by "shape" I mean the shape of the curve repre- 

 senting the electric field strength, or whatever other variable one 

 chooses to take, as a function of time at a point tra\ersed by the 

 wave) can be accurately reproduced by superposing an infinity of 

 wave-trains, of all frequencies and divers properly-adjusted ampli- 

 tudes, which efface one another's periodic variations, and in fact 

 efface one another altogether at all moments except during the time- 

 interval while the jiulse is passing o\er — during this interval they 

 coalesce into tiie pulse. Thence, the argument leads to the con- 

 tention that the actual [Hilse is made up of just such wa\'e-trains, 

 and the sa|)ient diffracting crystal recognizes them all and diffracts 

 each of them dui\' along its proper path. The problem is not new, 

 nor tlie answer; white light has long been diagnosed as consisting of 

 just such pulses, and the method of anahzing transient impulses in 

 electrical circuits into ilu-ir et|ui\,ilent sums of wa\e-irains has been 

 strikingly successful. 



The ajiplication of the mellKid to liiis case nf X-ra\' excitation 

 eiijo>ed one qualilati\e success. The spherical pulse tli\erging from 

 the place where an electron was brought to rest should not be of etjual 

 thickness at all the points of its surface; it should be broader and 

 flatter on the side towards the direction whence the i-leitron came, 

 thinner and sharper on the side towards the direction in which the 

 electron was going when it was arrested. Analyzing the jjulse, it is 

 found that at the point where it is broad and low, the most intense of 



