FAVOURABLE TO THE INVESTIGATION OF PARALLAX. 
351 
The Fourth Table. — This Table has been formed for the purpose of exhibiting the 
results from which, in conformity with the principles explained by Sir John 
Herschel, the existence or non-existence of parallax in the stars observed must be 
inferred. 
These results are the differences between two angles of position, the one observed 
at one period of the year, say the early one, and the other at another period (the 
late) distant from the former about six months. 
For reasons which will appear hereafter, it has not been deemed necessary to in- 
clude the distances in this table. 
In so delicate an inquiry as that of parallax, it may not perhaps be considered a 
legitimate course to pursue, to compare any results except those distant from one 
another by an interval of about six months, or in other words, results which follow 
one another immediately at opposite seasons. It is on differences so obtained that 
many therefore may be disposed to rely exclusively in discussing the questions raised 
by this investigation ; and such differences, when furnished by the observations, are 
accordingly placed first in the table ; but it is in truth only a choice of difficulties ; 
it has been found impossible in the case of some of the stars to obtain such differences 
at all ; and in others, the observations are so few, and their weight consequently so 
small, at one at least of the periods compared, that little reliance can be placed on 
the results ; it has been thought better therefore to combine in the case of eabh star, 
and in the manner already explained, all the observations made in all the years at 
the early period into one mean result, and all those made at the late period into an- 
other, and to insert their difference, leaving it to astronomers to attach whatever im- 
portance to the comparison they may think due to it. This difference follows in every 
case the more legitimate comparisons at intervals of six months. 
The four binary stars are excluded from this Table. 
The second column of this fourth Table contains the mean epoch : — in the case of 
the Jinal means, that is, the means of all the early and all the late periods, the decimal, 
following an enumeration of several years, is the mean of all the decimals attached 
to the years of the individual means or sets, from which the final means are formed. 
The third column contains the positions, obtained by using the computed weights 
of the results employed, as before explained. 
The fourth and fifth columns contain the probable errors and weights of these 
positions. 
The sixth column contains the differences between the positions, which follow one 
another in succession, and which have been obtained at opposite periods of the year ; 
or, in other words, the quantity, on which parallax depends, and to which the sub- 
joined remarks on the results of the observations refer. 
The plus sign is attached to the difference when the change in the angle takes 
place in that direction, which is in conformity with the hypothesis of a pdrallax in the 
stars observed; and the minus sign when the change is in the contrary direction. 
2 z 2 
