MAGNITUDES and COLORS of 176 EXTRAGALACTIC NEBULAE Joel Stebbins and Albert E

MAGNITUDES AND COLORS OF 176 EXTRAGALACTIC NEBULAE Joel Stebbins and Albert E. Whiteord Mount Wilson and Palomar Observatories - Carnegie Institution of Washington California Institute of Technology and Washburn Observatory Received November 6, 1951 ABSTRACT The magnitudes of 176 extragalactic nebulae from 9.2 to 18.2 mag. have been determined with a photomultiplier; of these, 97 have also been measured for color, and both magnitudes and colors are reduced to the International System. Together with a previous list, published in 1937, the results give magnitudes for 310 and colors for 187 different nebulae. Relations for reducing all results to a common system are derived. The new data indicate a correction of —0.2 mag. to the photographic magnitudes between 10 and 13 of the Harvard Survey. As before, the nebulae of Hubble’s types E, SO, Sa, and Sb average about the same color at C = +0.85 mag., International; the Sc’s are bluer at Cp = +0.47 mag. Owing to the high latitudes of the objects in the present list, they are not suitable for detection of space reddening in the galaxy. The excess reddening of the distant nebulae has been discussed in a previous paper. The present extension of the earlier photoelectric photometry of extragalactic nebulae with the reflectors at Mount Wilson1 was undertaken at the suggestion of Edwin Hubble and Walter Baade to provide additional standards of magnitude and color for faint nebulae. The observations were obtained in the spring of 1947, Whitford going to Mount Wilson for March and April, Stebbins for May and June. The installation and the method of observation have already been described in a paper on some of the fainter nebulae of the present program.2 The principal improvements on the work of 1937 have been the substitution of a 1P21 photomultiplier for a Kunz photocell; the rebuilding of the photometer to take focal diaphragms up to 19 mm instead of 10 mm in diameter, thus increasing the field from 2Í7 to 5Í1 on the 100-inch; and the reference of the measures to polar standards directly instead of by way of the Harvard visual magnitudes. At the outset we planned to get the magnitudes of nebulae correct to 0.1, but, after the work was well started, the suggestion was made that we might also measure some stars in Selected Areas to 0.01 or 0.02 mag. for additional standards. This expansion of the program, with another decimal place of accuracy thrown in, added some difficul- ties, but the enlarged program was carried through. The results for stars in Selected Areas 57, 61, and 68 have since been published.3 The present report includes the magnitudes of 176 nebulae, with the colors of 97 of them,'all reduced to the International scale. The observations were taken with the series of focal diaphragms listed in Table 1, where the corresponding fields are given for each telescope. When a diaphragm is men- tioned in the tables which follow, these field diameters serve to identify the telescope. We prefer to use fixed known openings rather than a variable-iris diaphragm. The results of the observations are in Table 2. The first column contains the NGC number. In the next two columns the galactic co-ordinates l and b are from the Lund

1 Stebbins and Whitford, Mt. W. Contr., No. 577; Ap. /., 86, 247, 1937. 2 Stebbins and Whitford, Mt. W. Contr., No. 753; Ap. /., 108, 413, 1948. 3 Stebbins, Whitford, and Johnson, Ap. /., 112, 469, 1950. 284

© American Astronomical Society • Provided by the NASA Astrophysics Data System MAGNITUDES AND COLORS OF NEBULAE 285 tables, interpolated from the NGC positions brought up to 1900. The type is by Hubble or Baade. In the next two columns the Harvard photographic magnitude BP g and the diameters are from the Shapley and Ames catalogue.4 The first diameter in the diaphragm column applies to the magnitude measurement; the second applies to the color. In the few cases where a rectangular diaphragm was used, the two dimensions are given, e.g., 6'8 X 2.'3 for NGC 3556; here there was no change for the color. When a diaphragm includes the whole nebula, the sky often contributes 80 per cent of the total light. Since the color is in most cases rather insensitive to the size of the diaphragm, a smaller hole for this part of the measurement gives a better nebula-to-sky ratio and higher accuracy. In spirals, however, the outer regions are always bluer than the inner TABLE 1 Focal Diaphragms

Diameter Diameter (Mm) 60-Inch 100-Inch (Mm) 60-Inch 100-Inch 0.7. 0Í3 O'. 2 9.1. 4'. 1 2'.4 1.5. 0.7 0.4 12.6. 5.7 3.4 2.7. 1.2 0.7 19.0. 8.6 5.1 5.0 2.3 1.3 ones;5 but we could not stop to make a detailed study of every nebula, even if that were possible. The magnitudes Pgv and colors Cp on the International scale were derived from the photoelectric measures by the relations

m Cp= +0 96+ 1.00C47, (i) Pgv=Pe +0.29Q, where C47 is the photoelectric color index with blue and yellow filters and Pe is the photoelectric magnitude measured in the clear, both corrected for atmospheric extinction and referred to standards of the North Polar Sequence. Relations (1) differ little from the revised ones later derived for the stars observed in 1947.6 When the color was not measured, Cp was assumed to be +0.86 mag. for types E, SO, Sa, and Sb, and equal to +0.47 mag. for Sc. The adoption of mean colors seldom leads to an error of 0.1 mag. in Pgp. When the field of the diaphragm is smaller than the major diameter of the nebula, Pgp is in parentheses; the difference Pgp—HPg is given only when Pgp takes in the whole nebula. Additional data are in the “Remarks” column. For instance, the first object, NGC 147, was observed on two nights in 1944 with average deviations ± 0.15 mag. and ± 0.00 mag. for magnitude and color, respectively. When an additional magnitude with a smaller diaphragm was obtained, the difference is, of course, always in the direction of numerical- ly greater magnitudes; e.g., for NGC 2685 the magnitude for 2Í3 was 0.24 fainter than for 4Í1. An interfering near-by star was either excluded (“Star out”), or if it was included with the nebula, its light was determined separately and subtracted from the total (“Star elim.”). Other remarks are self-explanatory. 4 Harvard Ann., 88, 43, 1932. 5 F. H. Scares, Proc. Nat. Acad. Sei., 2, 553, 1916; E. F. Carpenter, Pub. A.S.P., 43, 294, 1931; C. K. Seyfert, Ap. 91, 529, 1940; also unpublished photoelectric measures by the authors. 6 Stebbins, Whitford, and Johnson, op. cit., p. 471.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1952ApJ. . .115. .284S 8 4038.. 4036.. 4026. . 2694. 2685. 2681. 2655. 2537. 4102.. 4051.. 4038-9. 3990. 3953. 3941. 3898. 3665. 3627. 3623. 2768. 2712. 2693. 2654. 2639. 2549. 2536. 2532. 2460. 2300. 4138. . 4111. . 3998.. 3992. 3949. 3945. 3893. 3726. 3675. 3642. 3619. 3348. 3166. 3158. 3034. 3031. 2976. 2950. 280. 2535. 2403. 2314. 4125. . 3613. 3610. 3556. 3516. 3310. 3184. 3147. 3115. 2798. 2787. 2379. 2146, 224. 224. 206. 205. 4143.. 598. 224. 221. 221. 205. 185*. 147*. 147*. 185*. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem ♦The asterisksinthefirstandseventh columnsrefertothenotesatendoftable. NGC 211.2 206.8 216.3 256.0 256.0 211.8 142.8 101.7 141.1 101.1 135.8 105.3 120.7 129.3 140.7 110.3 109.5 101.1 122.9 150.2 108.0 108.7 118.1 110.8 122.3 134.2 134.2 124.6 134.4 123.0 135.9 126.7 164.9 155.1 123.7 152.5 106.6 102.7 102.1 111.4 103.1 106.9 103.5 103.6 107.4 112.6 113.0 113.6 113.3 105.3 113.2 108.5 110.2 114.3 145.1 102.9 110.5 121.8 146.8 165.0 117.6 99.7 94.6 98.7 96.1 89.5 87 ?9 89.0 89.0 88.9 88.9 87.9 89.5 89.5 89.4 89.4 89.0 +42.7 + 72.3 + 71.0 +54.8 + 62.7 + 60.7 +63.4 + 69.8 + 56.2 +41.7 +45.7 +45.7 +38.9 +42.3 +41.5 +40.0 +40.9 +38.8 +39.5 + 35.4 + 32.4 +55.7 + 75.4 + 59.7 + 66.1 + 67.4 + 55.3 + 65.5 + 65.3 +55.9 +55.3 +57.2 +43.0 + 57.0 +46.7 +56.7 +37.8 +41.3 +41.7 +41.6 + 41.5 + 33.3 +34.3 + 29.5 + 29.4 + 31.7 +30.2 + 23.2 + 28.2 + 25.5 + 73.3 + 72.6 + 63.7 +42.5 +42.5 + 65.0 +60.7 +67.3 + 51.6 + 65.9 +41.9 + 55.1 +40.1 +28.1 —14?0 -30.7 -21.3 -21.3 -14.2 -14.2 -14.0 -21.3 -21.7 -21.7 -21.8 -20.9 -20.9 p p Type Irr EO E2 S0p Sap SO SO SO E5 E5 SBO EO E3 E7 Sb SBO SBO SO SBb Sa Sc Sc Sb El E3 El | gP SBa SBa Sa Sc Sc Sb SO Sb Sa SO Sc Sb Sc Sa Sb Sc SBa SBa SO Sa Sa SO Sb Sc Sc Sb Sb E2 E2 Ep EP g SBO Sa E6 SO Sb SO SBb Sb Sc Sa Sep Sc Sc Sb Magnitudes andColorsofNebulae EPg 12“1 11.2 12.1 12.9 12.7 12.5 11.6 12.9 12.4 12.2 11.8 10.5 12.8 11.0 12.2 12.1 10.9 11.8 11.6 12.7 11.9 12.9 12.1 12.0 12.9 11.3 12.5 12.7 10.2 12.1 11.5 11.6 12.2 12.2 11.6 12.1 11.7 11.9 12.0 11.6 11.2 12.1 11.4 12.0 11.8 12.3 12.0 11.7 10.8 10.8 11.8 11.0 11.7 12.4 12.9 11.3 11.0 12.2 11.6 (4.5) (4.5) (4.5) 9.8 9.4 8.9 9.9 9.5 9.5 7.8 160 160 160 60 16 16 10 4.0 2.0 3.0 4.0 2.1 4.0 8.0 0.5 3.2 0.4 2.3 2.6 3.5 4.0 3.0 6.3 2.0 2.7 4.1 4.6 3.0 6.0 0.7 3.5 6.5 6Í5 2.0 3.5 2.3 2.5 2.2 8.0 2.0 7.0 5.0 2.6 8.0 5.3 8.0 0.7 1.0 1.0 8.0 1.7 7.0 .... 1.6 1.4 1.4 1.5 1.6 1.0 1.5 1.8 1.4 1.5 1.3 1.6 1.6 1.0 1.0 1.0 Diam. TABLE 2 40 40 40 40 10 ’¿’.Ó ’Ll '6 + 0.5 0.5 3 !8 0.6 0.7 0.7 0.7 3.8 2.0 2.5 3.2

TABLE 2—Continued

NGC Type HPg Diam. Diaph PSP PHPg • Remarks 5548. . 358?9 + 69' Sa 12?9 o:s o:s 2Í3 13I?3 +0m4 5557. . 30.3 + 68 El 12.6 0.7 0.7 2.3 12.2 — 0.4 5614.. 25.3 + 67 Sa 12.9 1.0 0.8 2.3 12.6 -0.3 5631.. 65.0 + 55 SO 12.5 0.9 0.9 2.3 12.5 0.0 Sky? 5672.. 16.6 + 66 Sb 1.3 1Í3 14.2 0^46 Bool*. 16.6 + 66 El 0.2 0.2 18.2 1.36 ±0.02, ±0.07 (3) 5813.. 327.5 +48 El 12.2 1.4 1.4 2.3 12.0 -0.2 5820.. 53.6 + 54.0 SO 12.8 0.7 0.3 2.3 2.3 13.4 1.15 + 0.6 Pgp±0.\1 (2), discordant 5831. 327.7 +47.6 E3 12.7 0.5 0.5 2.3 12.7 0.0 5838. 329.0 +47.9 SO 12.1 1.6 .... 4.1 11.7 -0.4 5846. 328.7 +47.4 E0 11.6 1.0 1.0 4A 11.2 -0.3 Companion included 5846. 328.7 +47.4 E0 0.7 13.7 Companion alone 5866. 57.9 + 52.0 SO 11.5 3.0 1.0 4.1 2.3 10.9 0.78 -0.6 Two stars elim., Pg ±0.04 (2) v 5879.. 59.2 + 50.9 Sb 12.1 3.3 1.3 5.7 ... 12.2 + 0.1 4Í1, 0.00 5899.. 35.4 + 56.2 Sb 12.4 2.3 0.6 4.1 ... 12.5 + 0.1 5921.. 336.2 + 46.5 SBb 12.5 5.0 5.0 4.1 ... (11.7) Star elim. CrBl*. 9.6 + 55.3 EO 0.4 0.2 16.8 1.13 ±0.02 (2), ±0.03 (3) 0Í2, 0.50 CrB2*. 9.6 + 55.3 El 0.4 0.2 17.3 1.15 ±0.04 (2), ±0.07 (3), 0Í2, 0.28 CrB3*.. 9.6 + 55.3 El 4 17.14 1.13 0Í2, 0.60 CrB4*.. 9.6 + 55.3 E3 4 17.09 1.13 0Í2, 0.60 5962. . . 358.8 + 49.0 Sc 12.5 1.9 1.0 1 11.9 -0.6 5970. .. 348.1 +46.7 SBb 12.4 3.0 1.0 1 12.4 0.0 5981. . . 59.4 +46.5 Sab 3 13.9 On edge; may be Sb 5982. . . 59.4 + 46.5 E4 12.5 1.0 0.7 3 2.3 12.3 0.85 -0.2 ±0.00, ±0.02 (2) 5985. . . 59.2 + 46.4 Sb 12.2 4- 0 2.0 7 11.9 -0.3 4Í1, 0.10 6015. . . 62.0 + 43.8 Sc 12.1 5.5 2.1 1 2.3 (11.7) 6 *45 Star elim., 2Í3, 0.14 IC 1183 359.1 + 43.0 El 7 14.9 6207.. 26.4 + 39.5 Sc .2.0 0.7 3 12.3 0.0 Star elim. 6217.. 78.0 + 33.4 Sc .1.8 1.2 1 11.8 -0.8 6503.. 67.4 + 30.5 Sc .5.0 1.0 5X14* (11.0). 0.57 6574. . . 9.7 + 14.0 Sb 1.0 0.6 3 1.3 12.8 0.76 + 0.1 7177. . . 43.8 -29.8 Sb 2.5 1.5 2.4 (12.0) 0.71 Sky? 7217. . . 54.7 -20 Sb 3.0 2.5 3.4 11.0 0.86 -o:6 Sky? 7217. . . 54.7 -20 Sb 3.0 2.5 1.3 (11.7) 0.92 Sky? 7332. . . 55.9 -30 SO 2.0 0.3 2.4 11.8 0.85 -0.8 Sky? 7339. .. 56.0 -30 Sc 2.4 13.0 0.84 Sky? 7448. .. 56.4 -39 Sc 11,8 2.0 0.8 2.4 11.9 0.39 +ó:í . Sky? 7457. .. 64.6 -27. SO 12.3 2.0 0.5 1.3 (12.6) 0.84 Sky?

NOTES TO TABLE 2 NGC 1471 185/ Diameter probably considerably larger than Shapley-Ames value. See comment in text. 3556 Rectangular diaphragm. 4244 Rectangular diaphragm. IC 783 1 t0 These four are called SO; they are probably dwarf members of Virgo cluster similar to NGC 147 and 185 of the local 4328 I group. All four look alike and lie near NGC 4321 (M 100) (comment by W. Baade). All are members of the Coma cluster. Only 4872 is listed in Shapley-Ames, but the dimensions and magnitude fit 4889, 4872 ] the brightest member. The catalogue values for 4872 aré listed under 4889 in the “HPg” and “Diam.,, columns of t0 r Table 2. The nebula here identified as 4872 is 018 south prec. 4874, the second-brightest member of the cluster. The 4889 I faint Anon member has the following rectangular co-ordinates relative to 4874: 5173 prec., 5157 south. Boo 1 Brightest member of the Boötes cluster (see Stebbins and Whitford, Mt. W. Contr., No. 753; Ap. J., Vol. 108, 413, Fig. 5, 1948). CrB 11 to f Four brightest members of Corona Borealis cluster (see ibid., Fig. 4). CrB 4) 6503 Rectangular diaphragm.

At the beginning of Table 2 are results on several members of the local group of nebulae obtained in 1944 with a photocell and filters7 giving colors C5 on about double the scale of International C. These measures were also referred to polar standards and reduced to the same system of Pgp and Cv. After NGC 598 in the list, all observations were made in 1947 and on one night only, unless otherwise noted. Summarized comparisons of our magnitudes in Table 2 with those of Shapley and Ames and with our previous ones of 1937 are in Table 3. Since the stars included in each 7 J. Stebbins, M.N., 110, 422, 1950.

© American Astronomical Society • Provided by the NASA Astrophysics Data System MAGNITUDES AND COLORS OF NEBULAE 289 mean difference are not the same ones, the three values under m are not necessarily con- sistent. The probable error r\ of a single difference indicates a good enough internal agreement in each of two series of the photoelectric measures. The systematic difference is to be ascribed in part to the decision3 to base the zero point of photoelectrically determined magnitudes on polar stars between 7.0 and 14.0 mag. rather than on stars near 6.0 mag., which presumably was the basis of the Harvard visual system which we used in 1937. By grouping the differences of 1947 in five means from 10.6 to 12.7 mag., we find m m Pgv — HP g = -0 4 + 0.12 {HPg - 10 0) ; (2) but four nebulae averaging 9.5 mag. are quite discordant from this relation, giving an observed mean difference of —0.05 mag. instead of the computed —0.46. In 1937 a mean correction of —0.1 mag. to the Harvard magnitudes brought them nearly enough into accord with our values at that time. We now conclude from the 117 stars of Table 3 TABLE 3 Systematic Differences

m No.

Pgp (47)-HPg 0.19 ±0.22 117 Pgp (37)-HPg • .08 ± .17 59 R^(47)-Pfo(37)... ■0.15 ±0.08 11

TABLE 4 Mean Colors by Types

so Sa Sb Sc

CP (37)... -K»0±0.12 (30) +0^86±0.10 (13) +0?84±0.12 (12) +0^59±0.18 (36) Cp (47)... +0.86±0.06 (32) +0.83±0.04 (8) +0.83 ±0.12(4) +0.82±0.12 (11) +0.47±0.08 (11)

that a correction of —0.2 mag. to the Harvard magnitudes from 10 to 13 will bring them into agreement with our present determination of the International scale from magnitude 7 to 14. From 17 nebulae suitably observed for color in both series we find the relation between Cp of 1947 and C2 of 1937 to be

Cp= +0“70 + 2.26C2 (3 + 0.01 + 0.10 (p.e.).

Using equation (3), we compare the colors for different nebular types in Table 4. Follow- ing each Cp in the table is the standard deviation for one color and the number of objects in the mean. The differences are scarcely significant except the one for Sc, which is probably caused by the larger diaphragms ordinarily used in 1947. Reverting to equations (1), we see that it requires a deviation of about 0.3 mag. from the mean of Cp to cause a change of 0.1 mag. in Pgp. Table 2 contains five nebulae of the irregular type, which divide sharply into two color types: +0.74 + 0.03 (2), and +0.27 + 0.04 (3). A similar division has been noted by E. Holmberg8 in his recent study of near-by systems. Although many of the brightest 8 Lund Medd., Ser. II, No. 128, 1950.

© American Astronomical Society • Provided by the NASA Astrophysics Data System 1952ApJ. . .115. .284S 8 14 13 9102 1 2 graph, oneisnotpreparedforHolmberg’sresult, whichwouldassigntwo-thirdsofthe much smallerfractiontothetotal.Holmbergfound anaveragecorrectionof0.06mag. ter. OnBaade’sillustrationshowingresolvedstars,thediaphragmusedwouldberepre- low surfacebrightnessesagainsttheskybackground isshown,however,bythefactthat where itwasaslargetheShapley-Amesdiameter. Thedifficultyofevaluatingthese total lighttotheareaoutsidecircle. for thelatteramajoraxisof14(5,aboutfourtimesthatgivenbyShapleyandAmes. Baade remarkedontheabnormallylowdensitygradientinthesesystemsandfound majority ofthecases,diameterwascheckedbyHubbleonMountWilsonplates, to Whitford’smagnitudesinallowingforthearea outsidediaphragminthefivecases the totallight1.2mag.brighterthanresultinTable2forShapley-Amesdiame- This figureforthediameterhasbeenconfirmedinHolmberg’sphotometry,whichfinds using asetofringswhichshowedthesizeavailablediaphragms. at leastaslargethatestimatedbyvisualinspectionofagoodoriginalplate.Inthe as thatofthewholenebula,itrepresentsameasurementanareawhosediameteris appreciable amounttothetotal.Ingeneral,whereamagnitudehasbeengiveninTable2 outer extensions,’which,becauseoftheirconsiderablearea,maycontributean fade graduallyintotheskybackgroundmustrunriskofmissingextremelyfaint outer regions,butfortheEnebulaenodifferencebetweeninnerandregionshas ing 22commontothetwoseries.ThereductionofC(37)(47)isgivenbyequation latitude effectwouldbeinsignificant. berg’s andatotallight0.7mag.brighter.Asimilar discrepancyfortheminordiameter Redman andShirleyfoundforM31amajordiameter 1.35timeslargerthanHolm- sented byacircle53mmindiameter.Evenafterinspectionofthiscarefullymadephoto- been detected. we cangettoInternational.Similarly,therearecolorsfor187differentnebulae,includ- A correctionof—0.1reducesthePg(37)tosystem(47),whichisasnear Of these,thereare222magnitudesfortotallight,including11nebulaeinbothseries. ford’s measurementsoftheseobjectsin1936.Theagreementissatisfactory: cussed separately.Alsothevariationofescbinpresentlistissosmallthatany there islittleeffectoftheredshiftoncolors.Excludedaremembers comes out: There are,however,tencolorscommontothetwolistsandforthesemeandifference with Holmberg’sresultsonnear-bynebulaeforacomparisonofthemagnitudescales. 60-inch telescope,thepresentlistofmagnitudeshasfiveobjectsalsocoveredinWhit- near-by systemsaretoolargetobecompletelycoveredbytheS'.6diaphragmon (3). Mostofthecolorsrefertoinnerregionswhich,forspirals,areredderthan Coma, Corona,andBootesclusters,forwhichtheeffectofredshifthasbeendis- 290 JOELSTEBBINSANDALBERTE.WHITFORD For thesamereasonoflargeangulardiameter,therearenotenoughobjectsincommon 2p p 10 11 13 12 14 9 HarvardBull.,No.895,1934. © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem M.N.,97,416,1937. Mt.W.Contr.,No.696;Ap.J.,100, 147,1944. Pub.Obs.U.Michigan,8,103, 1941.Ibid.,PI.VI. StebbinsandWhitford,Proc.Nat.Acad.Set.,20,93,1934. For nebulaeofnormaldensitygradient,however, thelightinouterareasaddsa The twodwarfellipticalsystemsNGC147and185areexceptionalcases.W. Attempts byanymethodtomeasuretheintegratedlightofobjectslikenebulaewhich In thetwolistsof1937and1947,310nebulaewereobservedinonewayoranother. Since mostofthenebulaeinTable4arebrighterthanthirteenthmagnitude, m Pg (47)-Pg(36)=+0±0.05(p.e.). p m C —Cho=-001+0.08(p.e.). p 1952ApJ. . .115. .284S 12 10 Hubble andBaadeformanysuggestionsintheplanningofthislastprogram,alsoto age ofthefaintouterfringe,buthomogeneoustreatmentallnebulaeagiven on statedcriteriafortheapparentdiametershouldproveuseful.Indiscussionsofnebular favorable anebula-to-skyratio.Completecoverageoftheouterfringesmustremainas Hiltner. thank Mr.HaltonC.Arpforfurnishingthegalacticco-ordinatesinTable2. them andtoMr.EdisonPettitforco-operationintheworkattelescopes.Wealso Mount Wilsonextendingovertheyearsfrom1931to1947.WeareindebtedMessrs. type, whetherlargeandbright,orsmallfaint. distance usingtotalmagnitudes,themostimportantquestionisnotcompletecover- the ideal.Untilthisratherelusiveidealisattained,however,workingmagnitudesbased the practicalobservationalconsiderationsofdodgingfieldstarsandavoidingtooun- of M31existsinthewidelydifferentvaluesfoundbyShapleyandWilliams © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The presentpapercompletesthereportonphotoelectricobservationsofnebulaeat The diaphragmschoseninthepresentmeasurementsrepresentaslightconcessionto MAGNITUDES ANDCOLORSOFNEBULAE291