CHAPTER I. 
instruments. 
The visual and photometric work was done with three different telescopes, 
of 6, 12 and 40 inches aperture, thus making it possible to follow the variable 
stars throughout their cycles, and to measure the magnitudes of all the comparison 
stars needed. The lack of homogeneity in the visual comparisons, resulting from 
the use of such different apertures, could not be avoided, but its harmful effect 
was kept within limits by the use of the photometric magnitudes of the com¬ 
parison stars. 
The 6-inch reflecting telescope, made by Brashear, is provided with driving 
clock, circles (the hour circle movable), and rotating tube, making zenith observa¬ 
tions comfortable, also making it possible to bring the line of the two stars com¬ 
pared parallel with the line of the eyes, a matter of prime importance in visual 
work. Standard stars from the Harvard and Potsdam catalogues, chosen between 
the 6th and 8th magnitudes, could be matched in brightness by the artificial 
star of the photometer. If brighter than the 7th magnitude the shade glass was 
used, its cell being hinged so that it could be turned into the cone of rays between 
the ocular and the flat. The visual limit of the 6-inch with a power of 150 is 
about 13.0 (see p. 10), but with the power of 40 used with the photometer it was 
not possible to make accurate measures of stars fainter than 11.0, while in practice 
the stars measured were between 8 and 10. 
The 12-inch Brashear refractor and the 40-inch Clark refractor of the Yerkes 
Observatory need no special description. Their visual limits are about 14.5 and 
17.0 magnitude, respectively. In the photometric work the greatest accuracy is 
secured by connecting with the 12-inch, standards between 8th and 9th magnitude 
with measured stars of about the 12th magnitude, then with the 40-inch to pass 
from the 12th magnitude to the faintest stars possible, about the 16th magnitude. 
THE EQUALIZING WEDGE PHOTOMETER AND THE DETERMINATION OF THE 
ABSORPTION OF ITS WEDGE. 
In pursuance of a plan for cooperation in determining standards for faint 
stellar magnitude, Prof. E. C. Pickering sent to the Yerkes Observatory in April, 
1900, one of the five wedge photometers which he had devised for the work. This 
was to be used with the 40-inch refractor in the measurement of the faintest stars 
included in the plan. The construction of the instrument is shown in fig. 1 and 
Plate 1. The tube T, carrying the ocular 0 , slides into the tailpiece of the 
telescope. At right angles to this is the tube C, carrying the essential parts of 
the photometer. The light from a one-candlepower incandescent lamp L shines 
through a minute hole in the diaphragm D upon a piece of ground glass G, forming 
5 
