At least two plates of each field were taken. In any case the two best plates were put on the “blink” comparator, and only those objects clearly nebulous on both plates were marked. The better plate was then placed on a Gaertner measuring machine. The nebulae and all the A.G. stars fainter than the seventh magnitude were measured in X and Y with the same screw. After an interval of a day or two the plate was remeasured. Settings were read to 0.01 mm, corresponding to about 0.87" on the scale of the plate. The two measures of a nebula differed but seldom in this unit, and if faint reference stars could have been used, a higher degree of accuracy could have been maintained throughout the work.
In order to orient the plate, two stars were selected with as large a difference in right ascension and as small a difference in declination as possible. The difference in X was then computed in millimeters, assuming a scale-value of 87.4″ per mm. The plate was placed in the measuring machine with the meridian roughly perpendicular to the screw, and adjusted with the tangent slow-motion until the setting on the two stars gave the computed difference to the limit of accuracy of the settings. This method reduced the constant of orientation of the plate to an almost negligible quantity.
Turner’s method (The Observatory, 16, 373, 1893) was used as the basis for reducing the measures. The six plate-constants for each field were determined from the five or six A.G. stars most symmetrically distributed about the optical center, and graphs were constructed from which the values of Y-Y₀ and X-X₀ were corrected. The Δα and Δδ were then added directly to the α₀ and δ₀ of the assumed center, and final corrections to the positions were read from graphs constructed from Turner’s formulae. Any A.G. stars not included in the determination of plate-constants furnished checks.
Since the distribution of the stars was about the same as that of the nebulae, it was hoped that the reduced positions of the latter would be of the same order of accuracy as those of the stars. For the 62 A.G. stars on the seven plates measured, the average difference from the A.G. positions was 1.0″ in either co-ordinate. The settings on the nebulae could be made with greater precision than on the stars, hence these results justify placing the accuracy of the nebular positions at about 2.0″ in either co-ordinate, except for such as were near the edge of the plates.
The positions are given for the epoch of the A.G. catalogues, 1875.0. Two of the N.G.C. nebulae had been measured by Lorenz from photographs made at Heidelberg. A comparison of his measures with the present measures shows the same second of arc in declination and the same tenth of a second of time in right ascension. The agreement is perfect to the last units used in the present paper, but as the nebulae, N.G.C. 7619 and 7626, are situated near the center of the plate, they cannot serve as a test of the accuracy of those near the edge. In several cases comparisons could be made with positions from visual measures, as given in the Strassburg Annals, Vols. III and IV. Here the agreement is not so good.
In one case, on the same night, positions of four objects were measured with reference to a certain star, which was, in turn, tied up to an A.G. star some distance away. The objects are given as N.G.C. 3550, 3552, 3554, and a nova which is designated as K₁₂. The photograph shows N.G.C. 3550 and 3554 properly placed with reference to the star, both in distance and in position-angle; there is nothing at all in the place given for N.G.C. 3552; K₁₂ is properly placed from the star, but is in the N.G.C. position for No. 3552. The position of the reference star is given as about 17″ too small in declination. K₁₂ is clearly N.G.C. 3552, and evidently the fourth object does not exist in the published position. As these objects were all in the same field of view in the telescope, one is at a loss to account for the discrepancy. A list of the comparisons is given in [Table II].
The descriptions indicate form, brightness, and size, and occasionally the location of a neighboring star. Wolf’s classification was used. It is, as he remarks, wholly empirical and probably without physical significance, yet it offers the best available system of filing away data, and will later be of great service when a significant order is established. One class was interpolated between g and h, and was designated g0. Brightness was estimated in the order B, pB, pF, F, vF, eF, eeF, and eeeF. The range is from Herschel’s Class II down to the limit of the plates. For several of the fields diameters were estimated to the nearest 5″. Otherwise the size was given in the order L, pL, pS, cS, S, vS, eS.
The classification given in [Table III] is illustrated in [Plate III], copied from Wolf’s engraving in Band III, No. 5, of the Publicationen des Astrophysikalischen Instituts Königstuhl-Heidelberg. The most striking feature is the great predominance of the classes e and f. These two classes form a continuous sequence from the brightest in the list to the very limits of the plates, where they are but mere faint markings on the films. Eleven are clearly spirals, and the spindles are unexpectedly common. These results are typical.
The frequency of the classes e and f may merely be a way of stating that the scale of the telescope is too small to show the ordinary structure, but it must be remembered that many members of these classes are pretty large and bright and that the gradation in the series is apparently continuous. As far as telescopes of moderate focal length are concerned, the predominant form of nebulae as we know them at present is not the spiral, but is this same “e, f” class, described as round or nearly so, brightening more or less gradually toward the center, and devoid of detail. The brightest of the class are probably Messier 60 and N.G.C. 3379. Their spectrum, as derived from objective-prism plates, is continuous, and is probably of the same type as those of the spirals and the globular clusters. A detailed study on an adequate scale of the brighter members of the class will throw considerable light on the problem of the small nebulae.
TABLE II