52 RADIATION 
On the basis of this simple theory, the present author, 
and later Hoelper [6], Tryselius [11], and Olsson [8], 
among others, have computed values for 6 and the 
total selective absorption from simple pyrheliometric 
observations. 
This method for treating actinometric observations 
has been discussed in some detail because considerations 
in the foregoing discussion are apt to provide an answer 
to the question which follows. Suppose we wish to use 
the simple actinometric measurements of Qm and Q, to 
obtain some idea of the scattering and absorbing prop- 
erties of the atmosphere (such a purpose is certainly an 
important justification for actinometric measurements 
in general), what is the accuracy that we should 
demand? It is perfectly clear from the simple theory 
presented here that, in order to make progress and ob- 
tain conclusions of importance, we are forced to intro- 
duce some simplifications in our assumptions concern- 
ing the laws governing atmospheric scattering. Such a 
simplification. has been made in assuming a simple 
exponential expression for the dependence of the scat- 
tering on wave length. This simplification represents a 
rather rough approximation, and because of this there 
is very little use in attempting to determine values of 
6 and a with an accuracy greater than about 5-10 per 
cent. This corresponds to accuracies in the values for 
the solar radiation Q,, and Q, of about 1-2 per cent. For 
determinations of the scattermg and absorption in the 
atmosphere by simple actinometric methods, this ap- 
pears to be the desirable accuracy.’ 
Actinometry with Regard to Biological Problems. 
The importance of solar radiation for a number of 
phenomena of biological character, such as plant growth 
and photosynthesis, and the health of man, has to a 
large extent led to the organization of actinometric 
measurements in connection with research concerning 
such phenomena. 
What accuracy is here required from the actinometric 
instruments? In trymg to answer this question, we 
must keep certain facts in mind. It is evident that, in 
general, the radiation which we are able to measure or 
record is very seldom the radiation that is effective in 
the biological processes under investigation. Neither 
the radiation on a horizontal surface, recorded for in- 
stance by a pyranometer, nor on a spherical surface, 
nor on a surface perpendicular to the solar beam as in 
pyrheliometers, is equal or strictly proportional to the 
radiation falling on a given plant or other organism. 
Furthermore, it is seldom of much use to try to con- 
struct a perfect model of a plant or organism since the 
effectiveness of the radiation is, in general, quite dif- 
ferent at different parts of the plant’s surface. In 
3. For a more detailed presentation of the methods for de- 
termining atmospheric turbidity on the basis of actinometric 
measurements along the lines indicated above, reference may 
now h2 made to a valuable treatise by Dr. Walter Schiiepp, 
Ptyjngied after the present article was written: Die Bestzm- 
mun, ler Komponenten der atmosphdrischen Tribung aus Ak- 
tinometermessungen. Inaugural Dissertation, Wien, Springer, 
1949. 
addition, a number of other factors, of which as a rule 
we have a rather incomplete knowledge, are generally 
effective in producing the observed results on, for in- 
stance, photosynthesis or growth. Therefore, if we ask 
for the equipment which ought to be recommended for 
meteorological stations when we have biological or 
agricultural needs in mind, it seems more important 
that the imstruments should be characterized by a 
certain simplicity and stability and that their results 
should be easily comparable with those from other 
stations, than that the instruments should be given some 
elaborate form in a rather vain attempt to imitate what 
can hardly be reproduced. Instruments which measure 
or record the radiation from the sun and sky on a hori- 
zontal surface and on a surface perpendicular to the 
solar beam thus seem to be able to satisfy more general 
needs. An accuracy of 5 per cent seems sufficient, 
provided that the instruments are not subject to sys- 
tematic errors or systematic changes. 
INSTRUMENTS AND MEASUREMENTS 
Determination of the Solar Constant for the Purpose 
of Ascertaining Its Variations. From what has been 
said above concerning the accuracy which must be 
demanded from the instruments for determining the 
solar constant, it must be concluded that none of the 
standard types of pyrheliometers in current use strictly 
satisfies these requirements. The two instruments which 
should be given first consideration are the compensa- 
tion pyrheliometer of K. Angstrém and the silver-disk 
pyrheliometer of the Smithsonian Institution (Abbot). 
Both have been subjected to rather elaborate investiga- 
tion and critical examination by various scientists and 
lately, in a very systematic way, by the International 
Radiation Commission. On the initiative of this Com- 
mission, Courvoisier [5] at the Observatory of Davos 
has made a complete theoretical survey of the principles 
of their construction, supplemented to some extent by 
experimental studies. | 
With regard to the Angstré6m compensation pyrheli- 
ometer, in the form in which it has been delivered by its 
manufacturers in Uppsala (Rose, now discontinued) and 
Stockholm (A. Lindblad), the following can be said 
of earlier as well as of more recent researches. 
As shown by the present author in 1914, the mstru- 
ment is subject to a small error called the “edge effect” 
arising from certain features in its construction. The 
“edge effect” is caused by the fact that one of the strips 
which constitute the sensitive part of the instrument is 
heated electrically throughout its entire length, while 
the other strip is illuminated by the solar radiation only 
to about 240 of its length on account of the screening 
of a diaphragm inside the instrument. The error which 
thus arises is of the order of 2 per cent. It is, however, to 
some extent dependent on the convection in the air 
over the heated strips. Taking ito account the condi- 
tions occurrimg in practice, we must conclude, from 
theoretical considerations supported by actual measure- 
ments, that the error arising from the edge effect may 
vary between the limits 2 + 0.5 per cent. 
