Physics. — “On the Effective Temperature of the Sun.” (2nd 
Communication). By H. Groot. (Communicated by Prof. 
H. W. Junius). 
(Communicated at the meeting of September 27, 1919). 
In a previous article of March 1919 it was demonstrated that 
the determination of the effective solar temperature by the applica- 
tion of Pranrck’s radiation formula to the data of ABBor, does not 
lead to a same temperature, independent of the considered kind of 
light, as estimated by A. Derant, but on the contrary that the value 
of 7 determined in this way varies systematically as 2. 
The meaning of the results so found will be examined in this 
article. 
It is necessary beforehand to define as strictly as possible what 
we mean by the term “effective temperature’, as the same meaning 
is not always attached to this expression. 
The reason that we cannot simply speak of the sun’s temperature 
is, first, that the sun has not the same temperature at all depths 
(thermodynamics show that for an extensive gas-mass — we 
must consider the sun as such — the temperature varies from 
layer to layer), and secondly that we cannot either indicate the 
temperature of a definite layer nor know the way in which the 
temperature depends on the distance from the centre of the sun. 
We can however find what temperature we should have to 
assign to the sun, so that, if it were an absolutely black body, 
it would behave im a definite respect exactly in the same way as 
we observe in reality. 
We may ask for example, what temperature an “absolutely black sun” 
must have if the position of maximum intensity in its spectrum is to 
be the same as in the real spectrum; or if the solar constant is to 
have the same value as the constant that has been determined 
experimentally. The first question may be answered by the aid of 
Wien's law; the second question by the application of the formula 
of STEPAN-BOLTZMANN. 
The temperature thus found is called “effective” temperature. 
Since, however, the sun is not an absolutely black body, we 
need not be surprised that the effective temperatures of the sun, which 
