617 
Conversely f and 7’ can be found when /, is known for two 
values of 2. 
The observed energy spectrum of the sun does not agree, 
however, with that of the black body, so that if we do apply 
PrarcK's formula for the calculation of f and 7’ from the experi- 
mentally determined /,, the value of 7 will depend on the place 
where we choose /). 
In table I the Ls* and 2°¢ columns give the values of 2 from 
whose corresponding /, the 7’ of the third column has been 
calculated. It seems to me that the application of PrancK’s formula 
to the experimentally determined energy spectrum of the sun’s 
radiation has not much sense, unless we could really consider this 
as almost agreeing with the spectrum of an absolutely black body — 
the criterion of which would consist in finding the same 7’ from 
arbitrarily chosen combinations of J). 
1 wrote already in my previous article: 
“The assumption that all kinds of light come to us from one 
photospheric surface, in other words that light of various wave- 
lengths should come from the same depth of the sun, appears more 
and more untenable..... 
If, however, in reality light of different wave-lengths originates 
from different parts of the sun, it becomes very questionable 
whether we shall be allowed to apply PrANcK’s formula, as we 
saw Derant do’. 
Instead of imagining one photospheric surface, as did Drranrt, 
we might try, what the supposition leads to that the sun is built 
up of a number of concentric “partial photospheres’’, each of them 
radiating as an absolutely black body, so that the total observed 
radiation is considered as built up of a number of partial 
radiations originating from different layers. In my previous article 
[ pronounced the expectation that on this supposition, considering 
the fact that it seems to follow from the work of SpiskerBorr and 
van Cirrert that in general we must look deeper into the sun 
for red light than for violet, the effective temperature would increase 
with the wave-length. 
This expectation has proved erroneous. And on closer con- 
sideration it was, indeed, unfounded. The effective temperature 
of a layer can, in fact, only be derived from the distribution of 
energy in its spectrum and the said result of SpiJkprBOnR and 
vaN Oirvert teaches us nothing about this. It is, however, worth 
while to examine the hypothesis of the “partial photospheres’’, 
because this may, perhaps, make if clear how the effective 
