318 
and has been succeeded by C. H. Hckles, 
formerly of the University of Missouri; J. S. 
Montgomery and T. G. Paterson have resigned 
as associate professors of animal husbandry, 
and R. C. Ashby as assistant professor of ani- 
mal husbandry, to enter commercial work; W. 
H. Peters, formerly head of animal husbandry 
of the North Dakota Experiment Station, has 
been appointed professor of animal husbandry; 
P. A. Anderson has been promoted from in- 
structor to assistant professor of animal hus- . 
bandry; J. C. Cort, formerly of Iowa State 
College, has been appointed assistant professor 
of dairying. 
DISCUSSION AND CORRESPONDENCE 
RED RAYS AND PHOTOELECTRIC EFFECT 
I wisH to call attention to an error which 
should be corrected as it is being repeated and 
found its way into such standard texts as 
Hughe’s “ Photoelectricity ” (Cambridge Uni- 
versity Press). Red light does not give a 
photoelectric effect with phosphorescent cal- 
cium sulphide, as the effect stops at the wave- 
length of about 4,200 Angstrém, as was shown 
by the writer.1 This result was later con- 
firmed at the University of Berlin. The re- 
sult is of considerable theoretical importance 
because the theory of the photoelectric effect 
which takes into account the necessity of a 
eritical energy content before the electrons can 
be shot off, shows that there will be a wave- 
length for each element beyond which no 
photoelectric effect will be produced. The ele- 
ment which gives the photoelectric effect in 
phosphorescent calcium sulphide is not known, 
but has been supposed by the writer to be 
sulphur as it is photoelectric for ultra-violet 
light and it was shown experimentally to give 
a photoelectric effect for wave-lengths longer 
than 3,200 Angstrom. This hypothesis could 
be established by showing that the photoelectric 
effect of sulphur ended at the same point as 
was shown for’ phosphorescent calcium sul- 
phide. 
When the writer began an investigation of 
the photoelectric effect of phosphorescent ma- 
1‘‘The Photoelectric Effect of Phosphorescent 
Material,’’ American Journal of Science, 1912. 
SCIENCE 
[N. S. Vou. XLVIII. No. 1239 
terial in 1910 at Yale University, it was sup- 
posed that the result obtained in 1909 by 
Lenard and Saeland at the University of 
Heidelberg was correct. However, it was found 
that the photoelectric effect of phosphorescent 
caleium stopped at about 4,200 Ang., which is 
a shorter wave-length than red light. Thus 
the result of Lenard and Saeland is incorrect. 
The error arose from confusing the effect of 
red light on the conductivity, which did exist, 
with that of the photoelectric effect which did 
not exist. In their paper in the Annalen der 
Physik, Lenard and Saeland described what 
they thought to be a new effect with red light 
which was called “ Aktinodielektrische Wirk- 
ung.” This effect differed from the photo- 
electric effect in that the test plate instead of 
charging up only positively, charged up both 
positively and negatively. It was thought that 
the long heat or red waves being more nearly 
comparable with the dimensions of the mole- 
cules affected them beyond the point where the 
photoelectric effect stopped. However, after 
working about a year on the effect of red rays 
on phosphorescent calcium sulphide, the writer 
came to the conclusion that no photoelectric 
effect could be obtained with red light and that 
the actinodielectrie effect was nothing more 
than an increase in conductivity such as had 
previously been known to exist for selenium. 
After the foregoing conclusion was reached 
a reeXamination of the original article of 
Lenard and Saeland showed that on account 
of a faulty construction of their apparatus the 
plate on which the material was placed was not 
completely. insulated from the accelerating 
and retarding fields, as is necessary when the 
photoelectric effect only is to be obtained. 
In order to confirm the conclusion, my own 
apparatus was later reconstructed at the 
Massachusetts Agricultural College so as to 
obtain both effects separately at will. It was 
shown with this apparatus that sulphur was 
both photoelectric and actinodielectric. The 
photoelectric effect required a high vacuum, 
but the actinodielectrice effect worked in addi- 
tioneat atmospheric pressure, the direction of 
the current depending upon the direction of 
the applied field. 
