57o SCIENCE PROGRESS 



representative line emitted by an element, or any function of 

 the frequency, is plotted against the atomic number of the 

 element, that is to say, a number which represents the posi- 

 tion of the element in the periodic table of the chemical ele- 

 ments, a smooth curve can be drawn through the indicating 

 points. Such a thing cannot be done if the atomic weights 

 are used instead of atomic numbers, and this alone shows that 

 we are now dealing with some very fundamental characteristic 

 of the atom. 



If it is the square root of the frequency which is plotted 

 against the atomic number, the smooth curve referred to 

 becomes very nearly a straight line. Whether it does so exactly 

 is a matter of exact measurement and correct interpretation. 

 If the curve is smooth, that in itself is one fact of first-rate 

 importance ; it will be an additional conclusion of importance 

 if a function of the frequency can be found, such as the square 

 root of the frequency, for which the curve becomes exactly a 

 straight line. 



The X-ray spectrum of each element contains a K series 

 consisting of four lines and an L series (which probably ought 

 to be divided into Li, L2, and other series) of more numerous 

 lines and longer wave-lengths. There are doubtless other 

 series as well, but the K and L lines are the most commonly 

 observed and measured. 



The^ stimulus which must be applied to cause an element 

 to emit its lines may consist of a projectile stream of electrons 

 of sufficient speed, or a beam of X-rays of sufficiently high 

 frequency. The limiting velocity in the one case and the 

 limiting frequency in the other have been the subject of recent 

 careful measurement and will be referred to later. 



The spectrometer which is used in analysing X-rays re- 

 sembles the ordinary spectrometer in general form, but differs 

 materially in details. No lenses or prisms can be used, since 

 they cannot refract or reflect the X-rays ; the collimater is 

 replaced by two narrow slits and the telescope by an ionisation 

 chamber, or by a photographic plate. The ordinary diffraction 

 grating, with its lines, say, 20,000 A.U. apart, is replaced by a 

 crystal whose atoms are arranged in planes which are regu- 

 larly spaced at intervals of two or three A.U. The minute 

 spacing is appropriate to the shorter wave-lengths to be dealt 

 with. 



