38o_ Dr. Herschei/s Account of the Changes 
one of those double stars of which I might ascertain the vacancy 
between the two stars, by estimating the number of diameters 
of the large one that would fill it up, I placed the star in the 
second class. However, by a measure taken with a micrometer, 
Oct. 22, 1781, the stars were found to be far enough asunder 
to come into the third class. By a mean of six measures, which 
were taken the first 18 months of my observing the star, their 
distance was 9" 32"'; and, by a repetition of estimations, it ap- 
peared, Dec. 22, 1781, that the vacancy between the two stars 
was not less than 4 diameters, and, when the air was tremulous, 
4 or 5. After an interval of more than 21 years, having omitted 
estimations by the diameter, as not very proper to be used with 
these stars, I wished to compare their distance with the former 
estimations ; and, with the same instrument and same magni- 
fying power that had been used before, the vacancy, Feb. 22, 
1803, amounted to 5 or 6 diameters of the large star; so that, 
certainly, an increase of distance must be admitted. 
The number of scattered stars in this neighbourhood, and the 
smallness of the star to which the relative situation of Rigel has 
been referred, render it probable that there is only a casual 
proximity, and no real connection, between these two stars. Nor 
can the change of their relative situation be accounted for by 
a parallactic motion of Rigel, although we should admit the 
small star to be without the reach of solar parallax ; for the 
effect arising from parallactic motion, would not only lessen the 
distance of the two stars, but would occasion a considerable dimi- 
nution in the angle of position, neither of which have taken place. 
As we have now the proper motion of Rigel, in Dr. Maske- 
lyne’s new Tables, we can no longer be at a loss for the cause 
of the change ; for, by a composition of the tabular motions in 
