Heidelberg, Universitätsbibliothek Heidelberg, Heid. Hs. 3695 EG 36,2 Gill, David; Royal Observatory Cape of Good Hope; Wolf, Max [Adr.]: Briefe von David Gill an Max Wolf: Brief von David Gill von Royal Observatory Cape of Good Hope an Max Wolf (Cape of Good Hope, 25.7.1906)

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1906 July 25. 190 .
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My dear Colleague,
I received your postcard of the 30th June
last with reference to the remarkable dimming of the sky
which you have observed at Heidelberg. I have looked into
the whole matter as far as the atmospheric conditions here
are concerned. On examining the photographs taken with the
Astrographic Telescope, we can find no diminution of the
number of stars obtained with similar plates and under
similar conditions with those of previous years.
With the McClean telescope, where we have been regular
-ly taking spectrograms for determination of motion in the
line of sight, we have with the same coil actuated by the
same number of accumulators what may-be called a fairly
constant actinic effect in a given time. This may be taken
to gauge the sensitivity of our plates,which vary somewhat.
Tor like density of the comparison spectrum we find a like
density
Professor Max Wolf,
Der Sternwarte,
Heidelberg.
Germany.