358 
found much smaller in the first period than later on, the day- 
observations give again the smaller value, and we may thus assume 
with great probability, that in 1864 this deviation must have been 
smaller for him than in the later periods. 
The very close agreement between the results of the day- and 
night-observations is of great importance from another point of view, 
as it proves, that an influence of the eccentric field illumination cannot 
be present to an appreciable amount in these observations. In 
examining this effect it is found, that it cannot be exactly the same 
for all the threads. Towards the side of the illuminating-mirror in 
the cube the effect becomes smaller, to the other side it increases, 
and the thread-intervals must therefore be found too small or tov 
large on both sides of the middle. The former will occur with the 
ocular pushed in too far and this independently of the side from 
which the field-light comes. In the original arrangement of the 
Leiden-instrument the direction of incidence of the field-light, which 
came from the side of thread VII, made in the middle an angle of 
2° 20' with the optical axis. At the extreme threads on both sides 
this angle was 14’ smaller or larger and here the relative effect 
was therefore 10°’, of the total effect at the middle thread. 
If the absolute effect for the middie tread was 2".0 (corresponding 
to the ocular being pushed in too far by 0.6 mm.), as must 
have been the case in the longitude-determination with Brussels, 
the relative effect for the extreme threads is found to be 0".2—0:.013 > 
which is just observable. By a comparison of the results CLL. 
and C/.W. the effect cannot be revealed; that of the results of 
day-and-night-observations in period I shows, that, in accordance 
with what was derived from other. facts, it was probably 
inappreciable. 
Our investigation thus makes it extremely probable, that the 
observers Kam and HENNEKELER in observing the stars which they 
observed also in declination, made an abnormal estimate of the time 
of transit over the iniddle thread, in exactly the same way in obser- 
vations: in the day-time as at night. Taking this into account very 
accurate values for the thread-intervals may be derived from their 
observations. The changes which have occurred in these intervals 
in the course of 5 years 1864—68 are found to be small as a rule, 
particularly in the case of the middle thread, notwithstanding 
frequent catching of the movable thread, by which the point of coin- 
cidence sometimes changed considerably. 
We subjoin the amounts found for the deviation in the transit 
over the middle thread for the various periods. 
