1452 
corona radiated less than 1/,,,,., of the ontput of the uneclipsed sun 
or °/, of that of the full moon. *) 
For observing the radiation during the annular eclipse we therefore 
decided to follow substantially the same plan, though with some 
alterations in the apparatus. This time the minimum would not be 
so low. From a close discussion of the Burgos results we presumed 
it to lie somewhere between */,,59. and */,5.,. So the galvanometer 
could be taken less sensitive, but, on the other hand, the steadiness 
of the zero could be improved and the period of oscillation shortened. 
Quickness of indication was, indeed, a very important condition, 
which not only the galvanometer but also the recipient of the radiation 
had to satisfy, if the minimum were to be observed exactly. 
At the observing station near Maastricht *), selected by the Eclipse 
Sommission of the Royal Academy of Amsterdam, the annular phase 
of the eclipse was expected to last less than one second. *) Our 
thermopile, used in Sumatra and Burgos, required 10 seconds for 
reaching a stationary temperature after being suddenly exposed to 
a constant source of radiation, and therefore would be too slow 
to catch the minimum, although quick enough to give the greater 
part of the radiation curve with sufficient accuracy. 
Description of apparatus. 
We determined on arranging two separate equipments: a rapidly 
working one, and a slower one, both suited for measuring the 
intensity of radiation from the first until the fourth contact, but in 
some respects complementing each other. The slower set of appa- 
ratus consisted of a thermopile (the same as used before), a moving- 
coil galvanometer of Siemers and Hatske with accessories, and 
suitable resistances. The thermopile was very carefully protected 
against all disturbing influences; it reacted only upon the radiation 
that passed through a long tube fitted with diaphragms and mounted 
parallactically, so as to be easily kept pointing towards the sun by 
means of a finding arrangement‘). We had ascertained by a special 
1) Proc. Roy. Acad. Amst. Vol. 8, p. 503, 1905. 
2) A preliminary account of the observations made by the Netherlands Expe- 
dition on April 17th 1912 is to be found im Proc. Roy. Acad. Amst. Vol. 14, 
p. 1195 (1912). Cf. also: NyLanp, “De eklips van 17 April 1912”, Hemel en 
Dampkring 10, 1, May 1912. 
3) According to J. Weeper, Proc. Roy. Acad. Amst. 14, 947, 1912. 
‘) A description of the instrument is given in: Total Eclipse of the Sun, May 18, 
1901; Reports on the Dutch Expedition to Karang Sago, Sumatra, N°. 4, “Heat 
Radiation of the Sun during the Eclipse”, by W. H. Jutrus (1905), 
