Survey of Northeast Greenland. 459 
passes almost unbearable. In the highest parts of Moskusoksefjeldene 
there are a large number of small valleys with water holes and a 
sporadically occurring boggy vegetation, so that the conditions in 
this very place are presumably favourable to the development of the 
mosquito larvæ. It must be borne in mind that in 1912 we did not 
come across mosquitoes in unpleasant numbers, neither at Danmarks 
Havn nor elsewhere on the low land. Nor did we in 1906—1908, as 
far as I know, come across such numbers of mosquitoes that we 
found it necessary to use mosquito nets or to take other measures 
for our protection. When we arrived at Daniel Bruuns Land in 1912, 
the season was too far advanced (end of August) to make it likely 
that we should come across mosquitoes in large numbers. 
Mirages. 
In the atmosphere of the Polar regions a number of optical 
phenomena occur — aurora polaris, halo phenomena, mirages — 
which even though they are not particularly restricted to the polar 
zones, yet contribute towards giving arctic and antarctic Nature its 
peculiar stamp. In this place I shall only mention the mirages, such 
as they appear most commonly in Northeast Greenland. 
In the course of the later expedition 1912—13 the mirages were 
subjected to a systematic investigation. This will be made the basis 
of a more elaborate treatment of the subject to appear later on. In 
this place I will only give a representation of the elementary optical 
principle, in order to enable the reader to understand the nature 
of reflection without having recourse to the technical literature"). 
The condition for mirage taking place is that the density of the 
air does not decrease evenly in an upward direction, but in jumps, 
so that one may speak of a bounding surface between two air strata. 
At this bounding surface a total reflection may then occur, and in 
that manner we get a mirage. According as the denser medium lies 
uppermost or nethermost, the “downward mirage” or the “upward 
mirage” occurs. 
The rays of light are not suddenly refracted in the same manner 
as in a prism, but are subject to a gradual deflection; for the sake 
of simplicity I have in the figures below represented it as being a 
reflection in the true sense of the word. Further I have, for the 
sake of distinctness, drawn the rays of light much steeper than they 
are in reality. The deflection to which the rays of light are subject 
7) J. M. PERNTER: Meteorologische Optik, Wien und Leipzig 1902—1910 has in 
chapters 3 and 4 “Luftspiegelungen und Fata Morgana und verwandten Er- 
scheinungen” given a detailed destription of the phenomena appertaining to 
this subject. 
