July, 1903.] 



KNOWLEDGE 



153 



Even in the matter of details, the agreement between 

 different observations is very often remarkable. Between 

 the two small, very near stars, ttj and iTo Cygni, in the 

 extreme north of the constellation, the luminosity becomes 

 slightly hollow towards the south. But this little, almost 

 insignificant, detail, which really exists — it can be seen 

 also on the j^hotographs — is to be found in all the 

 descriptions and drawings that I have examined. 



There is, therefore, no need to be in despair that we 

 shall never succeed — and even soon — in depicting in a 

 definitive manner, and in fair detail, the status presens, of 

 the visil)le galactic phenomenon. It would be a great 

 accom])lishment, and for two reasons. First, because it 

 is always important to record as faithfully as possible in 

 the storehouse of our observations all that nature pre- 

 sents to us ; it is, so to speak, the first duty of all who 

 study nature. And we can already see of what use such 

 a document may prove to future astronomers Is it not 

 strange that Ptolemy in his description of the Milky Way, 

 so comparatively exact, recoi'ded in the 138th year of 

 our era in " The Almagest," says nothing whatever of the 

 bright regions in Scutum ? We cannot surely admit 

 that all this region of the Milky Way was invisible 1800 

 years ago ; more probably the copyists have made an 

 omission. Nevertheless, if Ptolemy's description had come 

 down to us in greater detail and better preservation, we 

 should be in a position to-day to affirm that the phenomenon 

 had been invariable in its general form throughout 2000 

 years. It is not impossible that very slow changes in 

 brightness have come about in the case of most of the 

 stars situated in the same region of the sky ; they could 

 be detected by a comparison of observations made at very 

 distant epochs of time. Ought we not therefore, now, to 

 make an effort and devise to posterity a document for 

 such a comparison ? 



But another question is raised here. These efforts to 

 obtain a representation of the Milky Way, as it appears to 

 the naked eye, are they not useless in view of the enormous 

 jirogress of celestial photography in late years — progress 

 that does not seem to have yet reached its term ? 



I lielieve that this objection ought not to hinder us. 

 Certainly the gigantic eye of the telescope and the perfected 

 " retina" of the photographic plate are admirable instru- 

 ments, and in particular stellar photograjihs are of the 

 highest value. But at the same time let us not be 

 ungrateful; our eye, the simple human eye, has also its 

 advantages. Even the imperfection of the visual organ — 

 and this is a very interesting point— enables us to altain 

 one thing that photography is powerless to do : the im- 

 pression of the whole. It embraces and registers in a 

 single glance, in a single instant, everything that is 

 ])resented to it. The photographic plate on its side is, at 

 first insensible, it takes time . . . only it catches uj) again ! 

 It accumulates slowly, during consecutive hours, during 

 whole nights if we wish it, and in gathering up its treasures 

 of hidden light, it exposes to us stars that the greatest 

 telescope can scarcely reveal. . . . Oidy these are not, 

 so to speak, " ])hotographs of the Milky Way," though 

 they !ire often so called. They are ])hotogra])hs of stellar 

 groups, more or less numerous, in the Milky Way. The 

 optical phenomenon of the Milky Way evidently cannot be 

 rendered by a process of this nature. If finally we find 

 forming on the plates, spots analogous to the galactic spots, 

 they spring Iroiii other causes ; jiartly since the appearance 

 of such an image viewed from a certain distance produces 

 an optical illusion to our eyes comparable to that pro- 

 duced by the Milky Way. 



The ])hotographic process has errors of its own. The 

 light ot a, star gives rise to, not a luminous point, but a 

 little disk, which is far from being circular, esjiecially near 



the edges of the plate, and in the case of fairly bright 

 stars this can spread, and may " eclipse " the small stars 

 near. And if we examine such a plate carefully, especially 

 if we compare it with other plates which have been exposed 

 for a longer time, we often find that the small luminous 

 spots which are to all appearance but single stars, are in 

 reality groujjs of stars close together. 



A more serious drawback in celestial photography is 

 that we cannot yet succeed in obtaining anything more 

 than a narrow degree of uniformity. This is easily 

 explained when we consider the diversity of the instru- 

 ments, the condition of the atmosphere, etc. So far it has 

 only been possible to remedy in but an insufficient manner 

 this important obstacle, and at least for photographs 

 taken by different people at different times and places, it 

 comes aljout sometimes that two photographs of the same 

 celestial region present considerable differences, even where 

 care has been taken to bring the conditions into as much 

 accord as possible. The enthusiasm is much moderated of 

 those who a few years ago saw in imagination observations 

 which were entirely " mechanical " — the astronomers being 

 henceforth relegated to their desks. 



I do not wish to dwell here on the difference between 

 the actinic light of stars which alone affects the photo- 

 graphic plate and the light which imjjresses our eyes. 

 This difference, and the variety between the two kinds of 

 images which result from it, is enough of itself to establish 

 the need for visual observations. 



We might even find cause for astonishment that the 

 appearances of "'photographs of the Milky Way" and of 

 drawings should sometimes show so great a resemblance* — 

 and it probably proceeds from the fact that for the very 

 faint, inferior magnitudes, the great mass of the stars in 

 the Milky Way are nearly of the same specti-al type. 



I have thought it useful to enter somewhat in detail 

 into a question that seems to me of interest, especially for 

 naked-eye observers. In conclusion, I would like to say 

 something about the relation to each other of the optical 

 phenomenon of the Milky Way and of the physical 

 phenomenon — the Galaxy or the galactic system. But it 

 is a delicate question, where it is for the moment of 

 importance to affirm mithing and to accept results only as 

 provisional. 



What are the star.s which ])roiluce the optical 

 phenomenon ? 



In the first place, is there not here something l>eside 

 stars ? Nebulosities, sometimes brilliant ones, are shown 

 by photographv to be present in many parts of the Milky 

 Way ; do they not play in it an important role ? 



But there is little probability that this is so. Evidently 

 the rays emitted by these nebulosities are extremely ;ictinic. 

 That means that probably they make litth; or no impres- 

 sion on our visual organ. It is even possible to make a little 

 experiment in this matter. For this I chose a very brilliant 

 region of the Milky Way — between a and J Cygni. In 

 the small chart annexed (Pig. 1) I have traced as 

 accurately as possible the contours of the luminous spot 

 taken from tlie drawings of Heis, Houzeau, Panuekoek, 

 Boeddicker and my own. As might be expected, the 

 divergences are fairly great ; still the tracing on t; e whole 

 differs so widely from the contours of the celebrated 

 "America Nebula" which makes this region such a brilliant 

 one on the ]ihotographic plate (st-e Dr. Max Wolfs photo- 

 graph reproduced in tills nuinlier of Knowi.epoe^. that I 

 consider it established that even this nebula does not 

 sensibly affect the human eye. 



• A oompni-ison between Cliiii't I. of my "Voie Laet^e dans rbenii- 

 •spll^l•e boiviir' and Barnai-d's pliotogrnphs of tlie regions between 

 Altair and Scutum (Aslrophysical Journal, I.) is very instructive 

 fi-oin tliis point of view. 



