152 



KNOWLEDGE 



[July, 1903. 



on a smaller scale to the rays of light ; then the images 

 of many small and very close stars would each had their 

 own path of eommunicatiou with the brain, there would no 

 longer be a " confusion in the despatch." It is true that 

 the accumulation of luminous impressions which takes place 

 now would not be produced, but, instead, all the small 

 stars, down to a certain magnitude too feeble for our per- 

 ception — say, to the 9th or 10th magnitude — would be 

 seen individually in the sky. In such a case we should see 

 on very clear nights a great crowding of stars along a 

 somewhat irregular curve —300,000 stars in the visible 

 hemisphere ; it would be like the great atlas of Argelander, 

 the celebrated " Bonn Durchmusterung," spread out on 

 the celestial vault. But we should see nothing of the 

 " Milky Way " — except oa a somewhat misty sky which 

 might produce here and there some whitish spots and 

 bands. 



This word " Milky Way," the name given to the pheno- 

 menon which causes this grandiose optical error, gives rise 

 to some rather curious confusions. It establishes a funda- 

 mental distinction between the clusters of stars whose con- 

 stituents are sufficiently bright and sufiSciently separated 

 to be seen as stars (the Pleiades, for example) and gather- 

 ings of stars which produce on our eye the sensatioa of 

 "lacteal" light — although the two phenomena are identical 

 in their nature ; whilst the imperfection of our visual 

 organ makes us class together from their appearances, but 

 (juite wrongly, gaseous nebulae and the Milky Way. 



The visual organ is not equally good in all men, and 

 this has two results : — 1st. What is "Milky Way" for one 

 man is not so for another. 2nd. What is called " Milky 

 ^^'ay " in one quarter of the heavens is not so called in 

 another qviarter. 



Those who are not accustomed to raise their eyes in the 

 evening higher than the gas lamps in the street scarcely 

 know the Milky War at all, never having bestowed upon 

 it more than a ileeting glance. They are amazed that 

 anyone succeeds in seeing in it more than a baud, vaguely 

 and uniformly luminous, and seen with difficulty. 



The German astronomer Heis, whose piercing sight has 

 become famous, counted 13 stars in the Pleiades, where 

 even very good eyes see no more than " the hen with her 

 five chickens " : — 



" Quae septem dici, sex tamen esse soleat." 



Heis drew the Milky Way on his charts much larger 

 than other observers. Without doubt he saw its light far 

 outside ordinary limits. Searle has recently published his 

 observations of several " Scattered branches of the Milky 

 Way." Studying these attentively in exceptional atmo- 

 spheric conditions, he succeeded in some parts in following 

 these vague ramifications to regions near the galactic 

 pole. The astronomer who has perhaps best examined 

 the details of the Milky AVay, Dr. Boeddicker, of 

 Lord Rosse's Observatory, says that the background of 

 the sky is nowhere uniformly illuminated, and Mr. Back- 

 house, of Sunderland, has also written recently : — " I do 

 not tliink there is a uniformity of light in any part of the 

 sky at a reasonable altitude ; it is a question whether one 

 may not say there are streaks and patches of Milky Way 

 all over the sky." 



All who are familiar with the appearance of the sky 

 subscribe to this. Thus it would be possible to assert that 

 there is no liiuit to the Milky Way. It would be more 

 exact to say that, the word " Milky Way " hardly lieiug a 

 definable term, it does not seem possible to establish a 

 distinction between vaguelj' luminous regions which are 

 undoubtedly connected with the Milky Way, and those (the 

 nebulous region of Coma Berenices, for example) where the 

 relation with the galactic zone is at least very doubtful. 



If those who enjoy excellent sight see in general more 

 " galactic light" than others — as is natural — there are also 

 cases where the want of good eyesight produces the efiect 

 of galactic light where ordinary eyes do not perceive it. 

 A myopic person will see the Pleiades as a whitish spot, 

 as a galactic spot.* Aud this brings us back to what I 

 have just said, that " what is Milky Way for one person is 

 not so for another." This is a consideration which must 

 not be omitted when studying the Jelaih of the Milky 

 Wav on drawings made by different observers, for it will 

 exercise a certain influence everywhere where there are 

 groups or lines of stars on the verge of visibility to the 

 naked eye. To give yet another example, which anyone 

 can verify, the star l4 Aurigse, and the neighbouring small 

 stars (between i Aurigse and (i Tauri) are seen separately 

 in a very clear sky, but form a luminous line under 

 ordinary atmospheric conditions (or for short-sighted 

 persons). Such luminous lines or groups could thus 

 produce the outlines of factitious luminous spots on the 

 drawing ; and similarly for exceptionally good eyesight, 

 still feebler stars, grouped in other fashion, are at the 

 limit of vision, and so we have thus one cause of error. 



Another source of error and of divergence is to be found 

 in the circumstance that to compare the light of vague 

 galactic spots it is often necessary to pass the eye rapidly 

 from one spot to another, and thus the luminous im- 

 pressions accumulate, and are confused on the retina, and 

 we think we see details which do not really exist. 



I have wished simply to indicate several reasons why we 

 can never expect complete accord between the drawings of 

 the Milky Way made with the naked eye. There are 

 others and even more important ones. And it follows, as 

 it seems to me, that the main point is to fix first the 

 great features of the galactic figure, not paying too much 

 concern to the details, which are always uncertain, of 

 doubtful ramifications. For the same reason it seems to 

 me very desii-able that all those who apply themselves t-o 

 the naked-eye study of the Milky Way, so fascinating and 

 so useful, should add to their drawings descriptions as 

 detailed as possible. 



There is no need to despair of ever arriving at a sulficient 

 agreement. By no means. The causes of disagreement 

 which I have just indicated have only a very limited effect. 

 Even now, although we have only a very restricted number 

 of drawings of the Milky Way at our disposal, its con- 

 figuration can be settled, as to its major features, for a 

 considerable portion of the zone. This is partly the residt 

 of the excellent work recently published by Mr Backhouse 

 (" Publications of West Hendon House Observatory, 

 Sunderland," No. II., 1902). 



The differences which he establishes between his own 

 observations and those of other observers are often more 

 apparent than real f ; in many cases they could be easily 

 explained and eliminated. The important matter is to be 

 able to coui])are with each other a great number of drawings 

 made independently. I need only refer to wliat Mr. 

 Maunder has said in his book (" Astronomy without a 

 Telescope"), as the readers of Knowledge have no doubt 

 seen, as to the service to the science which the friends of 

 astronomy could render even without the possession of 

 any instniment.J 



* It is true that these- six stars would not certainly produce so 

 brilliant an effect, even for normal evesigbt, if their brightness were 

 not enhanced by the luminous background produci'd by the myriads 

 of stars on which they are projected. 



t In the majority of eases they are due to imperfections in tlie 

 reproductions. 



X I may perhaps be allowed to note here that " charts for inserting 

 the Milky Way " are to be had free of expense from the Editor of 

 Popular Asirottomi/, or from Dr. A. Pannekoek, Observatory, 

 Leiden. 



