=======+==============================+===================== | CENTER (1875.0) | NUMBER FIELD +--------------------+---------+-------+-----+------- | α | δ | Known | New | Total -------+--------------------+---------+-------+-----+------- I | 1ʰ 0ᵐ 30ˢ | +31°44′ | 21 | 57 | 78 II | 1 42 20 | 32 0 | 3 | 81 | 84 III | 11 3 54 | 29 27 | 8 | 178 | 186 IV | 13 37 10 | 56 21 | 21 | 52 | 73 V | 14 57 10 | 23 47 | 3 | 49 | 52 VI | 17 11 22 | 43 50 | 5 | 43 | 48 VII | 23 14 16 | 7 27 | 15 | 52 | 67 +--------------------+---------+-------+-----+------- Total| | | 76 | 512 | 588 -------+--------------------+---------+-------+-----+------- If very faint stellar images could be used for reference, this error could be largely reduced. It was necessary, however, to use stars from the catalogues of the Astronomische Gesellschaft for reference, and with the long exposures required for the faint nebulae the images of these stars were very large. At the edge of the plate, for instance, the arrow-shaped image of a star of the ninth magnitude would often be fully a minute of arc in length. It seemed inadvisable to make an exhaustive study of this magnitude-error, whence the alternatives were to use a restricted portion of the field or to sacrifice accuracy in the reduced positions. The second of these evils was chosen. The positions of the optical centers of images at various distances from the center of the field were determined empirically. Pairs of plates of a region were taken with apertures of 9 inches and with the full 24 inches and were compared in the Zeiss “blink” comparator. With the smaller aperture, and hence the smaller focal ratio, the images near the edge of the plates were sensibly round and small. Superimposed on the 24-inch images, they indicated where the wires should be set in measuring the larger distorted images. Trials were then made, measuring positions of A.G. stars all over the 24-inch plates, until a kind of technique was acquired. Judged by the aims in view and the results obtained, this empirical scheme fully justified itself.