+1.23 f o . 4 1 +0.23 +o.41
The stars contained in the present list have been taken from the list compiled by A . Kopf(A.N. Nr. 5375 and 5403). They have been designated primary comparison stars. Only the stars north of -17?0 are included, the more southerly stars being omitted on account of the difficulty of the measures for stars at the lower altitudes.
The necessary exposures have been made with the twin 3-inch cameras, of 2 I inches focal length, attached to the Bruce photographic telescope of the Yerkes Observatory. The photographic camera was stopped down to 42 mm, in order to equalize exposures taken with the photovisual camera, which was used at full aperture. The latter is corrected for the visual rays. I n front of the photovisual lens is placed an optically corrected yellow filter (Schott No. 4351), thickness 3.5mm. This filter cuts sharply at about 4950 A. Cramer Inst. Is0 plates were used, for the photo-visual, and Eastman 40 plates for the photographic. The efficiency of these lenses appears to be very high. Under the best conditions, for an exposure time of 2 0 minutes, or the standard exposure time adopted, the photographic lens, at 42 mm opening, records stars of magn. I 3.9 (corrected for atmospheric absorption), while the photovisual lens at full aperture will record stars of magnitude 1 2 . 1 on the photovisual scale. The minimum diameter of the images is 0.010 mm. For a zo-minute exposure, the diameter of Polaris is 0.23 mm for both cameras.
The magnitudes were obtained by photographing the Polar Sequence upon the same plate as the Eros stars, both before and after the exposure on the latter, which was made at the altitude of the pole. Scale plates, of which a number were used, served as the intermediaries of comparison. They were placed in contact with the plate being measured, film to film. On account of the small size of the images a moderately high power microscope was employed. The magnitudes of the stars of the Polar Sequence were taken from Trans. Int.