184 Dr. Herschel’s Method of observing the 
that may bias our judgment in assigning the comparative 
brightness of the stars : the different altitudes at which we 
view them : the state and situation of the moon : the time of 
the night with regard to twilight : the uncertainty of flying 
clouds : the twinkling and continual change of star-light, to 
whatever cause it may be owing ; I mean such changes as last 
but few moments, or at most but a few minutes : a return 
into the dark after having been writing by candle-light : the 
zodiacal light : aurora borealis : and dew or damp upon the 
glasses or specula when a telescope is used. All these, it must 
be confessed, are real difficulties, which it requires much at- 
tention and perseverance to get the better of. 
That the method is also laborious may be easily conceived ; 
for each star must at least have two other stars to be com- 
pared with, and even these will often be found not to be suf- 
ficient. To look out for such proper objects, and then to make 
the necessary comparisons for every star in the heavens, can be 
no easy task, especially when we remember the difficulties I 
have enumerated, to which every single estimation of compa- 
rative brightness is subject. This ought, however, not to dis- 
courage us from a work which has in view the investigation of 
a point of great importance ; and as I have already made a 
considerable progress, I shall give the result of my labour in 
small catalogues, of which I have joined one at the end of 
this paper. 
That these investigations are of the importance we have 
ascribed to them, will appear when we call to our remem- 
brance the great number of alterations of stars that we are 
certain have happened within the last two centuries, and the 
much greater number that we have reason to suspect to have 
