290 



SCIENCE 



[N. S. Vol. LIV. No. 1396 



tation, as it would have to be the same for all 

 substances and frequencies. The photoelectric 

 and thermionic works are known to agree to 

 within about one volt. To decide how far they 

 are identical needs better experimental evi- 

 dence than we have at present. The indirect 

 evidence for their substantial identity (that 

 is to say, within the limits of accuracy re- 

 ferred to above) is stronger at the moment 

 than the direct evidence. 



I do not think that the complete identity of 

 the thermionic work w and the photoelectric 

 hn^ is a matter which can be inferred a priori. 

 What we should- expect depends to a consider- 

 able extent on the condition of the electrons 

 in the interior of metals. We can not pretend 

 to any real knowledge of this at present; the 

 various current theories are mere guesswork. 

 Unless the electrons which escape all have the 

 same energy when inside the metal we should 

 expect the thermionic value to be an average 

 taken over those which get out. The photo- 

 electric value, on the other hand, should be 

 the minimum pertaining to those internal elec- 

 trons which have most energy. The apparent 

 sharpness of the threshold frequency is also 

 surprising from some points of view. There 

 seems to be scope for a fuller experimental 

 examination of these questions. 



I have spoken of the threshold frequency as 

 though it were a perfectly definite quantity. 

 No doubt it is when the condition of the body 

 is or can be definitely specified, but it is ex- 

 traordinarily sensitive to minute changes in 

 the conditions of the surface, such as may be 

 caused, for example, by the presence of ex- 

 tremely attenuated films of foreign matter. 

 For this reason we should accept with a cer- 

 tain degree of reserve statements which appear 

 from time to time that photoelectric action is 

 some parasitic phenomenon, inasmuch as it 

 can be made to disappear by improvement of 

 vacuum or other change in the conditions. 

 What has generally happened in these investi- 

 gations is that something has been done to 

 the illuminated surface which has raised its 

 threshold frequency above that of the shortest 

 wave-length in the light employed in the test. 

 Unless they are accompanied by specific in- 



formation about the changes which have taken 

 place in the threshold frequency, such state- 

 ments are of little value at the present stage 

 of development of this subject. 



Interesting calculations have been made by 

 Frenkel which bring surface tension into close 

 connection with the thermionic work w. 

 Broadly speaking, there can be little doubt 

 that a connection of this nature exists, but 

 whether the relation is as simple as that given 

 by the calculations is open to doubt. It should 

 be possible to answer this question definitely 

 when we have more information about the dis- 

 position of the electrons in atoms such as the 

 continuous progress in X-ray investigation 

 seems to promise. 



LIGHT AND X-RAYS 



One of the great achievements of experi- 

 mental physics in recent years has been the 

 demonstration of the essential unity of X-rays 

 and ordinary light. X-rays have been shown 

 to be merely light of particularly high fre- 

 quency or short wave-length, the distinction 

 between the two being one of degree rather 

 than of kind. The foundations of our knowl- 

 edge of X-ray phenomena were laid by Barkla, 

 but the discovery and development of the crys-- 

 tal diffraction methods by v. Laue, the Braggs, 

 Moseley, Duane, and de Broglie have estab- 

 lished their relations with ordinary light so 

 clearly that he who runs may read their sub- 

 stantial identity. The actual gap in the spec- 

 trum of the known radiations between light 

 and X-rays is also rapidly disappearing. The 

 longest stride into the region beyond the ultra- 

 violet was made by Lyman with the vacuum 

 grating spectroscope which he developed. For 

 a short time Professor Bazzoni and I held the 

 record in this direction with our determina- 

 tion of the short wave limit of the helium 

 spectrum, which is in the neighborhood of 450 

 Angstrom units. More recently this has been 

 passed by Millikan, who has mapped a number 

 of lines extending to about 200 Angstront units 

 — that is to say, more than four octaves above 

 the violet limit of the visible spectrum. I am 

 not sure what is the longest X-ray which has 

 been measured, but I find a record of a Zino 



