180 



KNOWLEDGE. 



[August 1, 1895. 



w 



Fig. 2.- 



tbe various groups. The question of bow many nominal 

 magnitudes correspond to stars which really belong to the 

 same system and are at the same distance, appears to be 

 capable of solution by this process, and other cognate 

 questions will have some light thrown upon them." 



To arrive at this result the simplest way would be to 

 classify, on photographs which show the fourteenth to 

 iifteentb magnitudes, all the stars accordmg 

 to their magnitudes, and to count the num- 

 ber belonging to each magnitude. But this 

 would entail gigantic labour, and would be so 

 difficult in practice, that there is no hope 

 of seeing it carried out for the present. In 

 the meantime I have attempted to compare, 

 at any rate as regards some regions of the 

 Milky Way, the connection between the result 

 of star gauges and star countings, and the 

 intensity of the general Galactic light. 



As regards the regions that I am about to 

 refer to, it can be safely assumed that in 

 general the greater or less intensity of the 

 Galactic light in a given district corresponds to the 

 greater or less density of the fainter stars, those inferior 

 to the ninth magnitude. (See for further particulars 

 Astronomiuhe Xiichrichten, No. 3270.) Now, if we consider 

 firstlv the zone included between K.A. 18b. 20m. and 

 19h. 40m. ; N. Decl. 0° to 6° (AquOa), it follows from the 

 evidence of a comparison of the descriptions and drawings 

 of the Milky Way by Heis, Troiivelot, Houzeau, Klein, 

 Boeddicker, Gould, Easton and Pannekoek, ■ that if this 

 region be divided into four eqixal parts, the part (A and 

 B) included between E.A. 18h. 20m. and 19b. Om. ; 

 N. Decl. 0° to G°, is poorer in Galactic brightness than 

 the opposite half (C and D) between R.A. 19h. Om. and 

 19h. 40m. ; N. Decl. 0^ to 6°; whilst the contrast is greatest 

 between A, where the Milky glimmer is the weakest, and 

 D, where the Milky Way presents considerable intensity. 



The number of stars of the first to 9-5 magnitudes can 

 be ascertained from the " Bonn Durchmusterung " of 

 Argelander for each of these four divisions and for each 

 semi-order of brilliancy ; the star counts by Celoria 

 [Piibhl. del Osserv. di Brera, XIII.) give the same data for 

 the totality of stars from the first to about the eleventh 

 magnitude. The following table is thereby obtained : — 



Table I. 



Way is seen to show itself about the ninth magnitude. 

 As regards the Celoria stars, the surplus of the stars in 

 the region where there is the greatest Galactic light 

 amounts to more than the half of all the stars found 

 on an equal superficies in the opposite region. The 

 correspondence shows itself a little earher, and is 

 much more marked when comparing the regions A 



XIX 



-Diagram iiidit-atiug tlie 

 magnitude 7'6 — 90. 



•tellar densitv for Stars of 

 (Argelander.) 



A correspondence between the distribution of the 

 relatively bright stars and the brightness of the Milky 



* See Easton, "The Milky Way in tlie Northern Hemisphere." 



and D, where the contrast of the Galactic brightness is 

 the strongest. 



Of the diagrams accompanying this article, Fig. 1 

 represents isophotic lines of the MUky Way as seen by 

 direct observation with the naked eye, and is from drawings 

 (unpublished) by Mr. Ant. Pamiekoek, of Leyden ; Fig. 2 

 is a diagram showing the numbers of the stars of magni- 

 tudes 7-0 — 9'0 (Argelander), the darkest tints corresponding 

 to the greatest density. No correspondence is shown with 

 the arrangement of the brighter or fainter regions of the 

 MUky Way. In Fig. 3 an analogous diagram is given for the 

 stars" of the lowest class of Argelander's magnitudes, 9.1 — 

 9-5, and the configuration of the Galactic image is already 

 beginning to manifest itself. The great rift between the two 

 main branches of the Jlilky ^^'ay, the condensation towards 

 8 and a, the feeble regions between i and v and 5 and 

 19 are well defined. The diagram from the stars counted 

 by Celoria, 1-11 magnitude (Fig. 1), shows these details 

 more clearly still. Fig. 5 is a reduced reproduction 

 (scale one-sixteenth) of the map in the Bonn atlas 

 corresponding to this region of the heavens ; thus all 

 the stars of the first to the ninth or tenth magnitude are 

 to be seen, so to speak, condensed — and a simple glance on 

 this map and on the drawings of the MUky Way wiU 

 suffice to prove — a conclusion arrived at also upon examina- 

 tion of the large celestial photographs — that a certain 

 number of relatively bright stars (1-9 magnitude) already 

 co-operate in the formation of the Milky Way properly 

 so-called. 



The results obtained for this region in the constellation 

 of the Eagle are confirmed in a remarkable way, if a more 

 extended zone is now considered, sufficiently distant from 

 the first, and situated in the constellation of the Swan, 

 between E.A. 20b. 20m. and 21h. 40m., N. Decl. 40° to 55°. 

 It contains one of the most brilliant spots (a to A Cygni) 

 and one of the most sombre spots (the " Northern coal 

 sack ") in the whole of the Milky Way. 



After having determined the details for which an accord 

 might be established between the greater part of the 

 observations of the Milky Way, I have divided the zone 

 into a certain number of trapezia, so that for each division 

 it becomes possible to give a value corresponding to the 

 brilliancv of the Milky Wav in this place. As a luminous 

 streak extends from f to 74*11 B.A.C., the band 47" to 48°, 

 having too much influence on the rather low value which 

 suits the division R.A. 20h. 52m. to 21h. 16m. ; N. Decl. 

 48' to '50% is not taken into consideration. The number I. 

 for the brUliancy of the Galactic brightness corresponds 



