154 



KNOWLEDGE 



[July, 1903. 



Let us then confine ourselves to the stars. Theoreti- 

 cally we mii^lit assert that all stars, however minut«, 

 exercise an influence on our retina — and, consequently on 

 the drawiui^ — provided that they are sufficiently numerous 



Fig. 1. — Drawings of a Galactic Spot (a— A Cygni) compared with 

 the contours of a brilh'aiit nebulosity. (The outlines of the drawings 

 have been indicated by dotted lines, ic. ; the form of the great nebula 

 is indicated by little crosses. The nuiubeis of the stars are those of 

 Flaiustecd (underlined), or of the General Catalogue.) 



and sufficiently crowded. The stars visibly separate to the 

 naked eye are naturally excepted. But in practice the 

 question may be thus put: What order of brightness 

 exercises the greatest influence ? 



Here, again, it is always the point to know if the stars 

 form groups that are crowded enough. But taken in 

 general, we can nevertheless show that the 1)rilliant stars, 

 at least to the 9th magnitude, do not exercise a preponderant 

 influence. This was demonstrated some years ago by 

 Plussmau, by means of a chart on which he had drawn by 

 gradations of colour a " theoretical Milky Way for Arge- 

 iander's stars " (to about 9-5 magnitude), in giving naturally 

 to the brilliant stars a greater part of the total light of 

 a certain portion of the sky than to the faint. This chart 

 — which was published in " Himmelskunde " (Freiberg, i., 

 Br. 1898) — sbows well a certain resemblance with the 

 course of the Milky Way, but also considerable diver- 

 gences. It is certain that these divergences are due to 

 feebler stars than those ia the Bonn Catalogue. Some 

 time ago I tried to elucidate this question by comparing 

 the distribution of the stars of different classes with the 

 distribution of the galactic light in certain regions of the 

 sky (Knowledge, August, 1895), and in a more recent 

 paper I pushed this enquiry further. 



In a very briUiant region of the Milky Way, to the south 

 of y Cvgni (Fig. 2), I have taken a surface measuring I^ 

 iu declination and 4 minutes in right ascension (co- 

 ordinates for 18550 : a = 20h 2m. 30s. to 20h. 6m. 80s. ; 

 S = 36^ + 37°. A single star (25 Cygni, B.D. 3907, 

 mag, 5'5 Arg. ; 5'2 H.P.) is visible to the naked eye in 

 this region. On a photographic plate by Prof Max Wolf, 

 of Heidelberg, I have observed and classified as carefully 

 as possible all the stars included in it, drawn with their 

 t stimated magnitude. They seem joined together on the 

 drawing* — 1760 are counted, classified as follows : 



* For particulars of these researches, I must refer the reader to my 

 recent article " The Distrioution of the Galactic Light, &c." in the 

 •'Terhandelingen de I'Atadcmie Boy. des Sciences des Pays-Bas, 

 VIII., 3. 



On the same area of the sky on which these 1760 stars are counted, 

 there is but one visible to the naked eye ; a short exposure gives 40. 

 (Compare Mrs. MaunJer's pliotograph reproduced in K>owledge, 



Estiuiated 



Mag. 

 7-6— 80 

 8-1— 8-5 



8 6- 90 

 91— 9 5 



9 6-100 

 101— 10-5 

 10 6—110 

 111— 115 

 11-6-120 

 121— 12-5 

 12 6— J 30 

 131— 13o 

 13-6-140 



Total 



Nnmljer- of 



Stars. 



1 



6 



6 



17 



42 



fjl 



103 



135 



134 



141 



1,S8 



229 



697 



1760 



Total light produced 

 by each class. 



243 



923 



584 

 1048 

 l'!39 

 1509 

 1613 

 1338 



840 



660 



472 



362 



697 



11,828 



These magnitudes are only estimated ; they cannot be 

 identical with Pickering's scale, for example, and the 

 limits cannot be constant. But in any case it is clear that 

 about half the total light (the share of each class of stars 

 iu the total light has been indicated in the last column of 

 the table, the brightness of a star of the first class 

 having been taken as unity) is due to the magnitudes 9-6 



-75 -80 -g.s '$.0 -9.s-/ao -io.f-//o-//.s -/2.0 -/2r o-o J'/. 



FiQ. 2.— 3j,,',n„tli part of the Sky: 1760 Stars. 



to Ho, or 9th to 12tli, and that even large errors in the 

 estimation do not sensibly modify this result; that neither 

 the relatively bright telescopic stars (6th-9tli mag.) nor 

 the exceedingly feeble stars (r2th-14th mag.) exercise any 

 considerable influence in the production of the optical 

 phenomenon of the Milky Way. A similar investigation 

 in a region of the Milky Way much less brilliant leads to 

 almost the same result. 



December, 1900 ; though, doubtless the negative would show a 

 greater number of stars.) 



If the entire sky were strewn as profusely with stars — though it is 

 plain that is far from being so — photography would show us 90 

 millions of stars down to magnitude 14. 



