20 6 Mr. Herschei/s and Mr. South’s observations of the apparent 
e Bootis continued. 
between the greatest and least among the single measures 
amounting to the enormous quantity of 16 0 io', and even 
among the mean results of whole sets of observations ex- 
tending to 10 or ii°. The closeness, and great difference 
of size and colour of the two stars, will partly account for 
this ; but if we compare our measures of this with those of 
Rigel, in which the difference of size is much more consider- 
able, and where the two stars are also very close ( the dis- 
tance being within g") we shall find reason to believe that 
some other cause than mere imperfection of vision, bias of 
eye, or error in judgment, must have operated. There can 
be no doubt but that, had the micrometer been purposely 
deranged i6° after any measure with which the observer had 
been tolerably satisfied, he could not possibly have avoided 
noticing the change on reviewing his measure. The remark 
annexed to the observations of April g, with the five-feet in- 
strument, shows that a much less change proved intolerably 
offensive to the eye. Refraction, acting differently on two 
stars close together, and differing so decidedly in colour as 
these do, might be expected to produce great alterations in 
their relative apparent situations, but unluckily this will not 
account for the particular changes observed. The point re- 
quires farther investigation. Meanwhile, the mean angle 
above given being concluded from 6 2 single measures, is 
probably near the truth. 
