INTRODUCTION. 
3 
(3) The stars to be compared should be in the same field, and 
(4) The interval in brightness should be less than half a magnitude. If this 
limit was exceeded the comparisons were weighted in the reductions, inversely 
as the interval. 
(5) Prejudice which would arise from anticipating the star’s expected changes, 
was avoided by postponing the reductions till the maximum or minimum was 
completed. The observing list was long enough so that the previous observa¬ 
tions were usually forgotten at the time of a comparison. 
(6) The comparison of too bright stars was avoided by reducing the aper¬ 
ture when necessary. 
(7) Tight in the eyes was avoided by using for recording a one-candlepower 
incandescent lamp, so shielded as to illuminate faintly a circle one or two inches 
in diameter on the record book. 
ESSENTIALS FOR GOOD PHOTOMETER MEASURES. 
Experience taught the necessity of close adherence to the following principles : 
(1) A sky free from clouds and of uniform transparency is the prime requisite. 
A “patchy” sky is a signal to stop photometric work. All observers agree to 
this in theory, but their adherence to it is a variable quantity, yet it is doubtless 
the largest single factor affecting the quality of the work. According to my 
experience a lack of transparency amounting to a perceptible haziness is liable 
to introduce a systematic error in the night’s measures. This is not the “local” 
error entering when distant regions, like polar and equatorial, are compared; 
but it shows in measures of a field limited to one degree in diameter. 
(2) Measures of a field should be repeated immediately, in inverse order from 
the first set, to eliminate progressive changes (a) in the transparency of the air 
and (6) in the brightness of the artificial star. Under (a), at least in the climate 
of the northern Middle States, progressive change in the transparency of the air 
is to be expected from hour to hour, and I suspect that this is true in all except 
a few favored regions. Admitting this, the duty of repetition in inverse order 
follows. The neglect of this apparently obvious precaution in most photometric 
work is difficult to explain or justify. 
(3) The real and artificial stars should resemble each other closely. This was 
possible in the present work with the 6- and 12-inch telescopes, the stars being 
usually indistinguishable, but it was not always possible with the 40-inch. 
(4) The stars to be compared should be in a uniform relative position. The 
practice of the writer has been to place them in a horizontal line, with the real star 
to the left of the artificial, and distant about 0.4 the space between the two images 
of the artificial star. Following the principle mentioned under (1) in “Visual 
comparisons” this uniform position seemed preferable to the four recommended 
by Professor Ceraski. 
(5) Nothing but the photometer lamp should be in the circuit from the storage 
cell supplying the current, and there should be no sliding contacts in the circuit; 
all should be soldered or screwed. The lamp should burn very nearly at its 
specified candlepower. 
