“26 A. W. Roberts.— Variable Star observing and [Oct. 28, 
stars as starting points from which to give relative magnitudes to 
all the other stars in the zone. 
It is well to do this though it is not absolutely necessary, as it 
gives a unity to all the observations and gives also the absolute 
magnitudes with reference to the whole heavens, and not the relative 
magnitudes with regard to the zone under consideration. Certain 
stars are therefore selected whose magnitudes differ by as nearly as 
possible half a magnitude. These stars are carefully compared with 
well known “ fixed ” stars. In Dr. Gould’s ‘ Uranometria Argentina ” 
a list of standard stars is giver. 
The observer is now ready to deal with all the stars on his paper, 
generally between one and two hundred in zones in or near the Milky 
Way. The two brightest stars whose magnitudes have already been 
assigned are taken, and all stars between them are compared and 
values assigned. Thus suppose the brightest comparison star is 
5° and the next 5°5. Any star very much nearer the brighter will 
be 5°2, and a star where the difference is almost imperceptible will 
be 5:1. This method of accepting values that have already been 
fixed by photometric measurement saves the observer all the trouble 
of deciding what quantity he will reckon equal to ;), and what to 7, 
As a rule, few observers can discern with absolute precision 54 of a 
magnitude. The usual “step” from one intensity to another is 
about 4+ of a magnitude. 
This means that when we notice that one star is just brighter 
than another, but that is all, the difference between them is one step. 
A difference very small but yet distinct enough, is two steps; a 
difference quite marked, three steps. 
Beyond this we need not go, as it is always possible to get two 
near stars separated by half a magnitude with which we can compare a 
star intermediate. The value of one step is constant for the same 
observer. My own determinations give a value a little less than } of 
a magnitude After the stars between 5 and 5°5 have been valued the 
observer takes up those between 5°5 and 6:0 and so on down to the 
9th magnitude. It is advisablealways to go over the whole lot again 
-by inter-comparison as a chance error will give a great deal of trouble. 
With micrometer measurements a series of values will decide 
the worth of any single observation. Not so in variable star work. 
If an error has been made it will take the observations of years 
to allay the suspicions aroused as to its variation. 
‘The next evening the observations are again gone over in the 
