92 I. P. KocH. 
gave the smallest collimation constant; it was consequently used as 
a middle thread. The thread intervals appear from the below reduction 
to thread No. 3 for д = 0. 
Thread No. 1 2 4 5 6 
Reduction 308.5 155.3 35.8 158.6 308.7 
The interval between the horizontal threads was about 2’. The 
threads were rendered visible at night by light thrown into the 
interior of the telescope through a pane off the horizontal axis of 
rotation. 
The special fitting up of the telescope with the large object glass 
and the inserted negative lens to shorten the focal length was hardly 
a success. It is true that the great intensity of the large field was 
very convenient, but the images of the stars were not well defined — 
the bright ones of the first or second magnitude had a wing on the 
one side — and consequently we did not attain the accuracy which 
would have been attained by an ordinary telescope, suited to the 
instrument. 
In the course of all observations made while the sun was visible, 
it proved difficult to protect the instrument against the insolation, 
which in arctic regions may become sufficiently powerful to produce 
a difference in the temperature of the air and the black bulb 
temperature of as much as 40° (Centigrade). In consequence the 
levels of a well verified instrument might oscillate in rather a dis- 
agreeable manner, as soon as the rays of the sun fell on the single 
parts of the instrument or on the iron table. The top and legs of 
the iron table were painted white, but this arrangement turned out 
to be insufficient, and it consequently became necessary to wrap up 
the table in a petticoat of white linen, and in this manner the diffi- 
culties as regards the iron table were surmounted. 
The instrument itself was more difficult to screen from the sun, 
because the screen covering it might easily disturb the manipulation; 
the insolation was here more dangerous, as there was no alteration 
in the inclination of the axis corresponding to the extension and 
contraction of the setting and legs of the level, and the consequent 
oscillation of the latter. Add to this that the level under the rays 
of the sun became so slow as to make the adjustment of the bubble 
uncertain. The difficulty was to a certain extent remedied by means 
of a little shade of white linen in a frame of bamboo cane. It was 
placed in a lamp holder, which was fastened to the edge of the iron 
table, in such a manner that the level with its setting and legs were 
in the shade. The lamp holder had a movable joint, which allowed 
