274 



KNOWLEDGE. 



[December 1, 1900. 



her owu automatic control, and to this the strain of long 

 continued observing was largely due. The substitute 

 for field illumination was simplicity itself; in the eye- 

 piece were four crosswires, and the brightest star avail- 

 able in the region was selected as the guiding star. The 

 eyepiece was drawn out so as to put the star out of focus, 

 and the spurious disk thus fonned was quadrisocted by 

 the crosswires — and kept quadrisected. Of course this 

 was sujjposed to be the clock's business, but here lay the 

 great source of disaster. If the spurious star disk 

 was not kept quite steadily on the intersecting wires, 

 but wandered off them, then the black wires became 

 invisible against the black sky, and all sorts of hiero- 

 glyphics might be drawn on the plate by the frantic 

 star before the intersection was found again. The 

 driving clock is a very valued friend, but his great value 

 and assistance cannot blind me to his idiosyncrasies and 

 failings, among which may be numbered a wearying 

 in well-doing that is more than occasional. He was 

 originally built to run for the short period that totality 

 lasts in a total eclipse of the sun, and not for the 

 hours that may be necessary in taking a star photograph. 

 When he had been running for some time, and had been 

 wound up two or three times, and was in a very good 

 temper, I might venture to leave him to go alone for a 

 period not exceeding 60 seconds. Twice, indeed, I have left 

 him to his own devices for a space of time nearly five 

 times that, but that proved to be a very rash action. 



The observer's duty, therefore, was to sit as comforts 

 ably as the position allowed with one eye glued to the 

 eyepiece, the declination slow-motion rod in the left hand 

 and the right hand touching the slow-motion wheel of 

 the right ascension circle. The best results were 

 generally obtained when a direct connection seemed 

 to be made between the eye and hand, and the con- 

 nections between these and the brain apparently 

 switched off. Usually it was fatal if I thought of what 

 I was doing; it was much better to think of something 

 else — something not exciting. 



In the photograph under consideration no j^erceptible 

 error in driving is to be seen; the guiding "star is per- 

 fectly round, and so are the stars situated within ten 

 degrees of the centre. During the exposure I saw many 

 of " The Teai-s of St. Lawrence." I hoped that some 

 would pass across the field of my camera, but I saw none 

 within that region, and none have been impressed on 

 the plate. 



The photograph was taken for the purpose of studying 

 the form and structure of the Milky Way, which may 

 be seen crossing the plate diagonally, and is well shown 

 throughout its greatest width. Much detail is seen on 

 the original negative that is unavoidably lost in a process 

 reproduction. The description is drawn from the original 

 negative, and therefore some of the structui-es pointed 

 out may not be recognisable on the accompanying 

 Plate. 



The scale of the original negative is almost exactly 

 that of Cottam's smaller Star Charts, and the field 

 photographed corresponds almost precisely to Chart 

 No. 22, the constellation Cygnus. 



The key-map, showing the brighter stars in the central 

 region, is traced from the negative, but the magnitudes 

 assigned are those of Proctor. As it will be at once seen 

 these arc very different from the photographic magni- 

 tudes, as might indeed be expected in such an actinic 

 region as the Milky Way. 



The large central star, which was used as a guide, is 

 a Cygui, and is situated in a t^aj). .or what might 

 by courtesy be termed a gap, in the Milky Way. The 



gi-eat star belt itself seems to be divided into five fairly 

 distinct regions. The preceding one of these is roughly 

 crescent-shaped, with 33 Cygui and 22 Cygni in the 

 northern and southern horns respectively, and 32, 8, 

 y and a Cygni lying ou its borders. A small 



The Constellation Cygnus. Key-map to Plate. 



horn, having its base resting on the stars 32 and 

 i5 Cygni, ap]*ears to point in a north-westerly 

 direction. The whole of this region appears to be 

 covered by a diffuse, but not uniform, faint cloud, which 

 gives the appearance of nebulositj', but which under the 

 microscope is seen to be. not nebulosity, but faint, fairly 

 well defined stars. y lies on the western border 

 of another region, smaller but more striking, since not 

 only are the faint stars aggregated so as to suggest a 

 nebulous bed, but brighter stars are also massed together, 

 giving the appearance of numerous and superimposed 

 layers of stars whose brightness diminishes with their 

 distance from us. More to the east again there is a huge 

 region, not seeming to differ in its composition (as far 

 as this negative can tell) from the two regions already 

 mentioned, except in the greater frequency of its locfil 

 aggregations of both bright and faint stars, and in its 

 more numerous channels where no stai's appeal- or only a 

 few sporadic ones. Such a channel, long and well- 

 marked, separates this region (which contains the stars 

 tt' and tt-, p, v, and s Cygni) from another, a fainter one, 

 and parallel to the first, still further to the east. 



But the most marked and interesting region of all, 

 is a small one on whose western border a Cygni 

 lies, and which contains the stars 55, 56, 57, 59, and 60. 

 To the unassisted eye this region appears on the negative 

 to consist of a dense nebulous patch, intersected by ex- 

 tremely fine streaks. Under the magnifier, the nebu- 

 losity to some extent resolves itself into faint and fainter 

 streams and bands of stars, these being again bound 

 together by still fainter bonds which ai'c not alwaya 

 resolvable into discrete stars. The streaks are some of 

 the spaces between the streams and uuresolvable bands, 

 where no star nor connecting stuff is seen. A most 

 beautiful and particular instance of this is to be seen 

 in a figiu'e of 8, formed of lines of stars and bands, 

 immediately following 60 Cygni, and which seems 



