2 
RESEARCHES IN STELLAR PHOTOMETRY. 
other assistance, made photographic copies of the forty Durchmusterung charts, 
then out of print, making possible the finding of the newer variables and the iden¬ 
tification of the brighter comparison stars; and of Prof. E. C. Pickering, who 
furnished charts and photometric magnitudes of many of the comparison stars. 
The instrument used so far was a 6.5-inch reflector by Brashear, but connection 
with the Yerkes Observatory for three months in 1898 and since January, 1900, 
has made possible the extension of the work to the determination of the minima 
of very faint stars, fixing the photometric magnitudes of the stars used for com¬ 
parison, and making photographic charts of the fields surrounding the variables. 
PLAN OF WORK. 
Two points were determined from the start: (1) To follow the variables as far 
as possible throughout their period; (2) to give special attention to the positions 
and identifications of the comparison stars. To carry out the first point, prefer¬ 
ence was given to circumpolar stars, which could be followed the year around. 
To fulfill point 2, the coordinates of the comparison stars from the variable 
were measured with a filar position micrometer, at first on the 6-inch, afterwards 
for twenty fields with the 40-inch for the faint stars. The completion of the 
24-inch reflector in 1901 made it possible to accomplish this object much easier 
and better by photography, so beginning in November of that year plates were 
taken to show the faintest comparison stars used. 
The addition of the Pickering equalizing wedge photometer to the outfit in 
1900 made it possible to complete the plan of work by adding (3) the determina¬ 
tion of the photometric magnitudes of the comparison stars, based on standard 
stars which had been measured both at Harvard and Potsdam. The publication 
of the Harvard catalogue and the approaching completion of the Potsdam work, 
including all stars north of the equator to magnitude 7.5, furnished the basis 
for this work, but it did more than that; it called for a minute comparison of 
the two magnitude systems in conditions of practical work. It is not enough 
to know the average differences between the two systems; the practical question 
is, What systematic differences will be found in standards selected for a particular 
field ? To anticipate the results discussed in Chapter XIV it may be stated here 
that the irregularities found in the systematic differences leave an outstanding 
uncertainty of about 0.1 magnitude in stars as bright as 7.0. 
ESSENTIALS FOR GOOD VISUAL COMPARISONS. 
Among the precautions kept in mind to insure good visual comparisons, 
the following were foremost: 
(1) The two stars to be compared were made parallel to the line of the 
eyes. To the writer this precaution was of the utmost importance, for if two 
equal stars were placed in a vertical line the lower would appear more than half 
a magnitude the brighter. The required position of the stars was easily secured 
with the 6-inch reflector by turning the tube in its cradle. 
(2) Two or three comparison stars were used at each observation if they 
could be found in proper distances and magnitudes, though this rule often con¬ 
flicted with the two following. 
