120 
NATURE 
[JANUARY 27, 1923 


from the surface of T by photoelectric action, and the 
energy of the swiftest of these electrons is given by the 
relation 4mv?=h(v — vp), where v is the frequency of the 
radiation and vg the threshold frequency of the metal 
T, 4 being Planck’s constant. The velocity v can be 
measured by applying a magnetic field perpendicular 
to the plane of the figure, when the electrons will be 
constrained to move in spiral paths, the axes of which 
are parallel to the magnetic field. Only those spiral 
paths the radu of which le within certain narrow 
limits will pass through the gaps S,,S,. Consequently, 
since this radius depends on the v elocity of the electrons 
and on the magnetic field, those electrons which reach 
the box I in a given magnetic field will have velocities 
lying between corresponding narrow limits. As the 
magnetic field is increased it will ultimately curl up 
the fastest electrons, so that their paths projected on 
Le, Friman . Crustal Grating. 
| 
+—}—_ +++ 
| | i} 
| | 
La Millikan | 
Shortest L line. Millikan —Per 
v Limif of He specttum. Richardson and Baxzoni. 
x L. Levels. from excitation vollage : 
(B,C Hughes C,.0,AL,Si, T, Fe, ee aah) Me 
+ lenisafion (or Radialion) Poleniials. - 
| He Li Be BC NO F NeNom, 
° e 4 6 s \o ze 

Fic. 3. 
to the plane of the figure lie along the circle TS, Sy. 
Any magnetic field greater than this will give rise to 
spirals which are too narrow to get into the box I, so 
that the magnetic field, which is just sufficient to stop 
the electron current into the box, will determine the 
velocity of the fastest electrons, and from this datum 
the equation quoted above enables the greatest 
frequency present in the radiation to be estimated. 
In this way we determined the end of the helium 
spectrum to lie close to the position 15°83 on diagram 
B of Fig. 1. The corresponding wave-length is about 
450 A. 
By 1916 Lyman had succeeded in measuring the 
wave-lengths of various lines extending to about 600 A 
by means of his vacuum grating spectroscope. This 
instrument of course measures the wave-lengths of 
the lines with precision, and is the most valuable 
weapon we have for research in this region. Notable 
advances have recently been made with it by Millikan, 
who has made several improvements in technique 
which have contributed to the success he has attained. 
These improvements include—(z) the production of 
NO. 2778, VOL. T11] 

6 to Ce A TE HB DH BMW M 36 

special gratings which are ruled with a light touch, so 
as to have about half the grating surface uncut, and 
thus throw nearly all the energy into the first-order 
spectrum ; (2) the employment of very high-tension 
sparks (some hundreds of thousands of volts supplied 
by Leyden jars and a powerful induction coil) between 
metal terminals very close together (0-1-2 mms.) in 
a high vacuum maintained by diffusion pumps. With 
this apparatus he has succeeded in measuring a large 
number of lines in the extreme ultra-violet spectra of 
the light elements lithium, beryllium, boron, carbon, 
nitrogen, oxygen, fluorine, sodium, magnesium, and 
aluminium, extending in the case of aluminium to 
136°6 A. This limit is shown at 16- +35 on Fig. 1, B. 
All these elements exhibit, under these conditions, 
characteristic line spectra which extend into the ultra- 
violet, and, roughly speaking, the spectra go further 
into the ultra-violet with increas- 
ing atomic weight of the elements. 
The spectra differ very much in 
character as between the different 
elements ; thus boron has but 
seven strong lines extending be- 
tween the limits 676°8 A and 
2497°8 A, whereas carbon has a 
very complex spectrum extend- 
ing from 360°5 A to 1335°0 
In fact the spectra of the ele- 
ments of odd atomic number 
such as boron tend to be simpler 
than those of even atomic num- 
ber such as carbon. The spectra 
of these elements in this region 
resemble the X-ray spectra of the 
heavier elements in this parti- 
cular, that they consist of groups 
of lines separated by very wide 
intervals, Thus with aluminium 
there is nothing between 144°3 
(La) and 1200 A, where a new 
spectrum starts which extends 
into the visible. 
There are good grounds for 
attributing the shorter wave-length groups of the lines 
of those elements in this region to the L characteristic 
X-rays of the elements. This will become clear by refer- 
ence to Fig. 3, which represents the square roots of the 
various frequencies plotted against the atomic numbers 
of the corresponding elements. The points encircled 
between atomic numbers 30-40 (Zn-Zr) belong to the 
L,, lines of the elements, the wave-lengths of which have 
been accurately measured by Friman by the crystal 
diffraction methods. These points are all practically on 
a straight line, which, if prolonged in the manner shown 
by the broken line, reaches the abscissa for atomic 
number 13 (aluminium) at a value of the ordinate 
which corresponds almost exactly to the line of wave- 
length 144°3 A, which Millikan found to be the longest 
in his group of aluminium lines in the far ultra-violet. 
This point is marked thus @ on the diagram. It is 
of course a long shot from zinc to aluminium, but we 
shall see later that we have other evidence of the 
legitimacy of the extrapolation. The other points 
marked @) refer to the longest lines, and those marked 
[=] to the shortest lines, of the spectra of the various 
38 a 
