1451 
Astronomy. — “The total solar radiation during the annular 
eclipse on April 17 1912. By Prof. W. H. Junius, 
Scheme of the investigation. 
The annular eclipse of the sun on April 17'", 1912, offered a rare 
opportunity for investigating the total amount of radiation due to 
the entire “solar atmosphere” i.e. to the complex of layers of the 
sun lying outside the level, generally indicated as surface of the 
photosphere. 
Every part of the solar atmosphere emits some proper radiation 
and scatters some photospheric light, and it is only natural to sup- 
pose that the lowest layers bear the greatest share in that radiation 
and scattering. Now, at a total eclipse the base of the atmosphere 
is always wholly or partly screened by tke moon; whereas during 
the annular phase of the eclipse of April 1912 even the lowest 
strata of the atmosphere all round the disk contributed to the 
remaining radiation. From the minimum value through which the 
remaining radiation passes at the instant of centrality one must be 
able to calculate an upper limit, which the radiation, emitted and 
scattered by the entire solar atmosphere, certainly does not exceed. 
Since a reliable determination of such an upper limit would afford 
an important criterion for testing fundamental ideas regarding the 
nature of the photosphere, the principal aim kept in view in devising 
our actinometrie apparatus was, that the minimum of the radiation 
curve should come out as sharply and definitely as possible. 
On former occasions (during the eclipses of 1901 in Karang Sago, 
Sumatra and of 1905 near Burgos) we measured the march of the 
total radiation by means of a thermopile directly exposed to the 
sun’s rays, without making use of any lenses or mirrors to concen- 
trate the beam. If circumstances had then allowed us to find the 
true. shape of the radiation curve, if would have been possible to 
calculate frum those data trustworthy values for the radiating power 
of successive concentric zones of the solar disk. *) Unfortunately the 
weather did not favour the Sumatra and Burgos observations; so we 
desired to make similar observations again. The apparatus had proved 
satisfactory, and sensitive enough to give measurable indications of 
heat even during totality ; for at Burgos a break in the clouds had 
permitted us to state that at mid eclipse the unscreened part of the 
1) W. H. Jvmus. A new method for determining the rate of decrease of the 
radiating power from the center toward the limb of the solar disk. Proc. Roy. 
Acad. Amst. 8, 668, 1905; Astroph. Journal 23, 312, 1906. 
