zé 
353 
Thread Clamp West Clamp East WE + i cee 
PTATSDEEAL 254 SAT BEDAN HpA EAT A 
II +All „ —All ke 7 +All „ 
UI +Alll ,, —AIIl _,, El J-AII „ 
v —AV+(1—p)A' LAV — pA’ ij LAV ,, 
VI —AVI cite he NV i +AVI „ 
VII —AVII „ +AVII „ 3 AVL 
The thread-intervals must therefore in each case be corrected by 
half the amount of the retardation (or acceleration), but the retardation 
itself cannot be determined, not even as to its sign, without making 
arbitrary suppositions, and, if in half the cases the bisection was made 
before m and in the other half after m, the retardation has no 
influence on W + E and remains completely indeterminate. 
We had thus come to the conclusion, that it would be hardly 
possible, even from the large material of fundamental stars, to derive 
accurate values for the reduction to the middle thread, when we 
discovered that in the last two periods a considerable number of 
transit-observations were available, which were not combined with 
observations of declination and which might throw light on the 
problem before us. A separate investigation of these observations 
not only revealed the true nature of the anomaly, which appeared 
to be entirely different from our former supposition, but*at the same 
time showed that its influence could also be completely, or at least for 
by far the greater part, eliminated in the remaining periods. This | 
investigation may be now detailed. 
As already mentioned, it had been found that neither a priori 
nor a posteriori any ground existed for separating the two periods 
Vb and VI. For the period Vb the corrections to be applied to the 
provisional intervals V had been calenlated, but naturally from these 
could be derived those other ones which would have been found, if 
the preliminary intervals VI had been used as the basis. 
The observations of non-bisected stars in the now extended period 
VI, Jan. 1866—1868, may be divided into 4 classes: (1) fundamental 
stars 1866—April 1867, (2) fundamental stars April 1867—1868, 
(3) stars observed in 1867 which had been used in the longitude- 
operations with Göttingen, (4) observations for the determination of 
the longitude Leiden —Brussels. Each class comprises between 50 
and 100 observations in each position of the instrument for each 
of the two observers. The longitude-determination with Brussels was, 
23 
Proceedings Royal Acad. Amsterdam, Vol. XIX 
