Z 452 1 
XIX. On the periodical Star a Herculis ; with Remarks tend- 
ing to establish the rotatory Motion of the Stars on their Axes. 
To which is added a Second Catalogue of the comparative 
Brightness of the Stars. By William Herschel, LL.D. 
F. R. S. 
Read June 9, 1796. 
In my first catalogue of the comparative brightness of the 
stars, I announced u Herculis as a periodical star. The pre- 
cision of the characters introduced in that catalogue is such, 
that the smallest alteration in the lustre of the stars may be 
discovered, by a proper attention to their expressions : the va- 
riation in the light of a Herculis is, however, pretty consi- 
derable, and cannot easily be mistaken, when strictly com- 
pared to a proper standard. The star most conveniently si- 
tuated for this purpose is z Ophiuchi ; and as I have had no 
reason, during the time of my observations, to doubt the uni- 
formity of its lustre, I have made use of it in the following 
comparisons ; which seem to be sufficiently decisive, with re- 
gard to the periodical variations of the light of a Herculis. 
Other stars besides z Ophiuchi have also been consulted ; 
but the unsteadiness of their light would draw me into diffi- 
culties, which at present it will be proper to avoid. For this 
reason I only give the following table, which will be found to 
contain at least four regular changes of the alternate increase 
and decay of the apparent lustre of our new periodical star, 
deduced from a comparison of its brightness with that of 
z Ophiuchi. 
