Survey of Northeast Greenland. 253 
refraction of the meridian aperture have been particularly favourable 
seems evident’). 
In case one wishes to make sure against the effect of the re- 
fraction of the meridian aperture one must measure the temperature 
of the air immediately in front of the objective of the telescope, 
which is not so very easy with an ordinary thermometer. On the 
Danmark-Ekspedition I made use of AssMANN’s aspiration psychro- 
meter for this purpose. The Germaniaexpedition, however, did not 
bring aspirators for the thermometers, and it has consequently hardly 
been possible to read off, with any certainty, the temperature in 
front of the objective. However, as BÔRGEN and COPELAND do not at 
all mention this difficulty, it seems probable that they have simply 
read off the temperature in the thermometer locker, which was to 
be found directly on the north wall of the Observatory. 
There are in the series of observations of the Germaniaexpedition 
several circumstances showing that BORGEN and COPELAND have not 
been able to avoid rather considerable systematic errors. Whether 
the refraction of the meridian aperture is one of these it is, at the 
present moment, not possible to decide, among other things because 
the individual temperature readings under the meteorological ob- 
servations have not been published. This much, however, is clear, 
that the discrepancy in the observations of the Germaniaexpedition 
cannot be explained solely on the hypothesis of a strong refraction 
of the meridian aperture (The determination of latitude at Muschel- 
berg, where the observations were performed in the open air, gives 
with the upper culmination of the sun ф — 75°11’32”.9 whereas the 
lower culmination gives ф = 75°11'16’.0 — Besslian refraction). 
BORGEN and COPELAND interpreted the discrepancy in a way which 
led to the computation of the above-mentioned factor of correction, 
0.952, of the Besslian refraction. A fuller treatment of the problem as 
to whether a factor of correction of this kind is fully justifiable will 
be found in the section “Astronomical Refraction” рр. 136—146. Here I 
shall only remark that the observations of the Danmark-Ekspedition 
may hardly be said to offer any support for the justification of a factor 
of this kind, and also that the observations of the Germaniaexpedition 
at least contain one circumstance which speaks against applying the 
factor of correction. 
On August 5th, 6th and 8th, 1869 BORGEN and COPELAND per- 
!) In the Observatory at Danmarks Havn throughout the summer I lifted the whole 
roof off, so that the Observatory only consisted of four whitewashed walls. 
Nevertheless the temperature of the air in front of the objective of the tele- 
scope might be more than one degree higher than that of the air outside the 
Observatory. 
