Changes that happen to the fixed Stars. 183 
lustre, much more must that change be evident in this case, 
where the stars are separated by lines instead of commas. 
When two stars differ so much in brightness that one or 
two other stars might be put between them, and still leave 
sufficient room for distinction, they become partly unfit for 
standards by which the lustre of other stars can be ascertained. 
But as proper intermediate stars sometimes cannot conveni- 
ently be had, we are often obliged to retain them ; and in that 
case I distinguish them by a line and comma -, or by two 
lines, as 32 — 41 Leonis. A difference which exceeds those 
that are expressed by the above marks, I denote by a broken 
line, thus for instance, lb 2 g Bootis. It would be 
very easy to give a more extensive signification to lines by 
adding cross marks to them, such as, -J- — j-} rfi H t y~ 
&c.; but in estimations that are to ascertain the brightness of 
stars, such expressions would rather throw us back again to 
look for imaginary differences, resembling those which have 
been rejected in the old system of magnitudes. On the con- 
trary, the marks I have introduced admit of so precise a defi- 
nition, that they cannot possibly be mistaken : a point denot- 
ing equality of lustre : a comma indicating the least perceptible 
difference: a short line to mark a decided but small superiority: 
a line and comma, or double line, to express a considerable 
and striking excess of brightness ; and a broken line to mark 
any other superiority which is to be looked upon as of no use 
in estimations that are intended for the purpose of detecting 
changes. 
In a foregoing paragraph we have said that this method of 
ascertaining the lustre of the stars was difficult and laborious. 
The difficulty consists in avoiding the various causes of error 
