in printing apply here also. We have lately made, at the Lick Observa ory, a few negatives of the Orion nebula, under good circumstances. The exposures have been made by Mr. SCHAEBERLE and myself, and the plates have been developed by Mr. BURNHAM and Mr. BARNARD. I have made a comparison between these negatives, the prints by Mr. COMMON and Mr. ROBERTS, the General Catalogue of Stars in the Nebula by Professor G. P. BOND, and an unpublished drawing of the Nebula, by Dr. J. F. J. SCHMIDT. A brief summary of the results of the comparison may be of interest, as illustrating the performances of different telescopes, and as exhibiting the excellencies of the photographic methods. It is known that Professor BOND'S catalogue of stars in the nebula required several years' work. It includes nearly every star visible in the Harvard College telescope down to the 15 or 16 magnitude on Professor BOND'S scale (which calls a star 17-18 mag., which is 15 magnitude. on ARGELANDER'S scale). The following table contains notes of the comparison: List of some of the stars of BOND'S General Catalogue of Stars in the Nebula of Orion, which have been identified in the photograph of Mr. ROBERTS. [Usually no account is taken of stars brighter than 13.0 in BOND'S catalogue. BOND's magnitude, 17-18, corresponds to ARGELANDER'S 15 magnitude]. STAR. BOND'S MAG. 122 175 178 211 212 10.8 {Compare in the photograph 122 (10.8) with 176 (10.8). 122 is very much fainter. 13.9 175 (13.9) is about equal to 122 in the photograph. 11.5 {178 (11.5) is about 14 mag. in the photograph. Compare 13.1 13.1 it with 326 (11.5), 705 (11.5) and with 175 (13.9). 13.9] is not shown in the photograph. It is immersed in nebulosity. 13.9 14.8] [216 222 241 260 14.2 270 276 13.1 14.8 These two stars are just visible in the photograph. They are not visible in the L. O. neg. exposed 58m, but they show in exposures of 100 minutes and upwards. 13.9 In the photograph 288 is considerably brighter than This star is not shown in Mr. ROBERTS' photograph, as it 13.3 15.0] I am not sure that this star can be seen in the photograph. 434 seems somewhat less bright than BOND's average 11.5 mag. Between 423 and 508 the photograph shows a star 14+ mag. not in BOND. This star just shows in L. O. negative 58m exposure. There is a star in the photograph closely s. p. 478, which is not in BOND's catalogue. It is not in any of the L. O. negatives up to 97m exposure. 11.5 {The photograph shows 583 considerably less bright than 13.9 11.5. Compare 326, 1051, etc. 605 and 610 are shown in the photograph as one star. They are shown in the L. O. negatives about of an inch apart, which illustrates one of the advantages of a long focus. 380 [413 419 14.2 583 587 605 13.9 { 762 772 [779 783 In the photograph 703 is considerably brighter than 693. 786 13.9 13.9 15.6] { 13.9 This star is quite bright in the photograph and in some of the L. O. negatives. It is absent or very faint in other L. O. negatives. Variable? Not seen in the photograph nor in any of the L. O. negatives. 13.9 787 13.3 In the photograph there is a star 14+ mag. between 786 and 847 not observed by BOND. I do not find it on the L. O. negatives. Does not show in the photograph. It is not seen in L. O. negative exposed 58m, and just shows in that exposed 97m. 9.2 {is 9.2 and 953 is 9.3 mag., according to BOND, but in the photograph 953 is considerably brighter than 888. 883 13.3 888 893 13.1 1051 11.5 14.8]} 14.2 Not seen in the photograph. It does not show in the L. O. negative exposed 58m, but it is measurable in that exposed 97m. Between 953 and 1015 are two stars, 14 mag. or less in the photograph, not observed by BOND; these are outside the limits of the L. O. plates. I am not sure if it is shown in the photograph; just visible in L. O. negative exposed 58m. 14.2 The positions in BOND are wrong, but both stars exist and 13.9 are in the photograph. 13.1 13.1 (These three stars are of the same mag. in BOND, but in the photograph 1029 is much fainter than I011; 1023= 1029 in brightness. 13.1 { Between 1015 and 1051 the photograph shows a star 13-14 mag., not in BOND'S catalogue. This region is outside of the L. O. negatives. The general result of the comparison is, that Mr. ROBERTS' print contains substantially all of BOND's stars, and it is probable that the negative shows a considerable number of fainter stars. I find that the Lick Observatory negative exposed 58 minutes, shows practically all the stars of Mr. ROBERTS' print. If there is any advantage, it is on the side of the print. The Lick Observatory negative exposed 97 minutes shows more stars than the print. It is probable that the latter negative and that from which Mr. ROBERTS' print was made, are about equal, as far as showing faint stars is concerned. That is, from 80 to 100 minutes exposure with the 33-inch telescope, will give about the same stars as 205 exposure with the 20-inch reflector. So far, the advantage is with the large aperture, as it should be, and the advantage would be even more apparent, were it not for the great thickness of the Lick Observatory object-glass. When we come to compare the extent of nebulosity depicted, the advantage becomes enormous in favor of the short-focused reflector. By examining the reproductions of Mr. ROBERTS' negatives of Orion given in Knowledge for May, 1889, page 148, I judge that 15 minutes' m exposure with the reflector is about as effective in showing the nebulosity of Orion as 60m with the refractor! If we take a nebula with a continuous spectrum (like the Andromeda nebula), and do not use orthochromatic plates, the advantage in favor of the reflector would be still greater. The pregnant remark quoted by Dr. KONKOLY"Jedes Fernrohr hat seinen Himmel"—is well illustrated by these comparisons, and they point a very practical moral. The comparison with Mr. COMMON's enlargement shows that his print gives about the same amount of nebulosity in 37" as is given by the Lick Observatory telescope in 97", rather more than less. The faintest star shown by Mr. COMMON's print is probably 378, about 14.8 mag., according to BOND. All of his stars are shown on the Lick Observatory negatives of the shortest exposure (60m or so), as would be expected. We have so far made no shorter exposures suitable for comparison. When it is considered that this print is an enlargement, the definition appears very fine, and the extent of the nebulosity would probably be even greater, if the picture had been made on the negative plates now in use, which are probably considerably more sensitive than those employed by Mr. COMMON in 1883. An examination of a few negatives lately made at the Lick Observatory gives the following data regarding the performance of the photographic objective (a = 33 inches, ƒ = 570 inches), on stars. A negative exposed 811⁄2 minutes on the clusters in Perseus shows 202 stars in an area of about of a square degree, in which the Paris picture of 1884 (probably not on the most sensitive plates) exposed 50 shows 77 stars. A negative of the cluster 20 Vulpecula exposed 40 gives everything in Professor SCHULTZ'S map, and something more; that is, it shows stars fainter than 13 mag. of BESSEL, probably as faint as, or fainter, than 13 mag. of ARGELANDER. The Potsdam 13-inch photographic refractor gives a 13 mag. star in about 20". The star close following the nebula in Zyra is a typical 13.2 mag. star. In 15 and 20" exposure this star is just visible, in 30 it is measurable, in 60" the star is very plain. In 60m the nebula itself just begins to make a complete picture with the 33-inch telescope. * Professor PICKERING has been kind enough to determine the photographic magnitude of this star. In a letter dated December 2, 1890, he writes: "I have had a photograph of the star in Lyra, which you mention, taken with an exposure of 20m in the 8-inch telescope, and a photograph of the Circumpolar Region on the same plate with the same exposure. This enabled the star in Lyra to be compared with the three stars Nos. 489, 506 and 543 in Table XI, p. 138 of Vol. XVIII, of the Observatory Annals. The results for the magnitude of the star in Lyra were, respectively, 13.2, 13.0, 13.4. It seems therefore to have about the magnitude 13.2 on the scale of Table XI, just mentioned." = PHOTOGRAPH OF THE CLUSTER M. 34 G. C. 584. A negative of the cluster M. 34 (2h 35m + 42° 17') was taken on Dec. 14, 1890 in a rather hazy sky with an exposure of 60". It has since been compared with the map given by Mr. O. A. L. PIHL in his Monograph (Micrometric examination of Stellar Cluster in Perseus. Christiania 1869, 4to.) The general agreement is excellent. There are, however, three cases of obvious disagreement between the negative and the map which it may be worth while to record. It is to be noticed that these cases all refer to the position or magnitude of the smaller stars (10 mag. or less) and that Mr. PIHL gave comparatively little attention to these.* It is therefore probable that the discrepancies referred to are accidental. Still, they are recorded as below. I. Near the stars nos. 35, 36 Mr. PIHL gives 3 others of about II magnitude. In the map the two northernmost of these point very nearly to Star no. 4. In the negative their direction intersects the line 4-1 somewhere near its middle point. II. In the map there is an 11 mag. star near no. 66, which we will call a. The line 66-a points (in the map) to Star 24. In the negative there is a faint star 13-14 mag, in the situation of a. There is also a star II mag. about 17" further north which is not given in the map. III. The magnitudes of the small stars on the line 66 to 79 are quite different in the map and in the negative. I may note a star given in the map at 2h 33m 15; + 42° 1' which the negative shows to be double (60°, s=3" mags. 11-13 estimated.) Mr. PIHL's observations were made with a small refractor of 34 inches aperture. Within the polygon bounded by the lines 69-47-59-68-79-75-69 he has mapped 14 stars, while the negative shows 27. This is a small increase under the circumstances and shows that this cluster is not very rich in the smaller stars. E. S. H. MOON-NEGATIVES TAKEN AT THE LICK OBSERVATORY, It may be of use to give here a list of the negatives of the Moon taken, up to this time, at the Lick Observatory with the 33-inch photographic lens. It is intended to continue making such plates until a pair of satisfactory negatives has been secured for every few hours of See his Memoir, p. 12. |