D O C U M E N T 4 5 J U N E 1 9 2 0 1 8 3
45. From Adolf Smekal[1]
Vienna, 5 June 1920
Highly esteemed Professor,
Perhaps you still remember that, during the discussion of my photophoresis re-
port at the Berlin
Colloquium,[2]
I asserted at the time that the Stokes-Cunningham
law of falling bodies was one of the best-secured laws of physics, owing to the
splendid agreement between the particle radii drawn therefrom and those from
Ehrenhaft’s
[3]
“optical” size
determinations.[4]
You, Professor, rightfully countered
then that the theory of Brownian motion was actually merely grounded on proba-
bility assumptions, and that the lack of agreement with the particle radii calculated
from Brownian motion did not speak favorably for this law of falling bodies.
Well, not long ago, Dr. E. Norst demonstrated in a talk before the Viennese chap-
ter of the G. Phys. Soc. that all the previous results from “optical” size determina-
tions are
useless.[5]
Since I am thus proven wrong, in your favor, Professor (which
with a bit more humility I might perhaps have been able to foresee!), I consider it
my duty to communicate the details to you immediately. Mrs. Norst found the fol-
lowing errors:
1.) The irradiation curves are in p[art] wrongly calculated; the number of points,
from which they are determined, is too small to exclude arbitrary factors.
2.) Instead of the arc-lamp spectrum (= glowing carbon + arc), G. Laski, J.
Parankiewicz, and M. Schirmann used the carbon-arc spectrum
(!);[6]
Snow, whose
measurements they used, had carefully screened out the
carbon![7]
3.) In applying the physiological theory, König’s basal perception curves, which
relate to the solar spectrum, must be
used.[8]
Laski had neglected to perform this
“Sun” conversion.
4.) Even in Laski’s work (without her having noticed it, partly also as a conse-
quence of computational errors), many different particle radii belong to a single
wavelength (albeit, the colors are of various saturations), so the equivocal corre-
spondence between color and size (or the rate of fall, which according to the obser-
vations exists qualitatively in any case) does not obtain.
Mrs. Norst avoided the above errors by using Parankiewicz’s irradiation curves
for sulfur and calculated the relation between particle radius and physiologically
effective color identification, on the one hand, neglecting whether the intensity dis-
tribution in the arc-lamp spectrum was the same as in the solar spectrum, and on
the other hand, taking into account the Sun : arc-lamp ratio, which had been de-
fined for this purpose by
Kohlrausch.[9]
In both cases, in general, larger particle
radii resulted; but here too the difficulty mentioned under (4) arose (lack of a defi-
nite assignment of color to size), so that “optical” size determination is presently
completely unusable and hence all the conclusions hitherto based upon it as well.