May 1, 1897.] 



KNOWLEDGE. 



121 



lie upon a " great lane," which may be traced far to the 

 north-west, and which here curves round to the east before 

 it loses itself. The extremest southern extent of the 

 nebula abuts in similar manner upon another lane, running 

 an undulating course from east to west. The marvellous 

 thinning out of the stars towards the south-western corner 

 of the plate reminds us that we are here on the edge of 

 one of the most remarkable inlets which invade the Milky 

 Way. 



Our next photograph, the left-hand one of the pair 

 between pages 50 and 51 of Vol. XVI., shows a wonderful 

 advance, due partly to a more prolonged exposure^eight 

 hours as contrasted with three — and partly, no doubt, to 

 an improvement in the plates and in the manner of their 

 development. The lens is the same as that employed 

 before, but the photograph has been enlarged more than 

 three diameters from the original negative. Several of 

 the " dark lanes " are distinctly traceable, but now the one 

 which seemed to separate between the northern and 

 southern parts of the nebula appears to dive under it. 

 The nebula itself is greatly extended, the "keyhole" or 

 " lemniscata" has all but disappeared, drowned in nebular 

 light, and the southern portions of the nebula extend their 

 milky light far over regions which appeared to contain 

 nothing but star points in No. 1. 



No, 3, the right-hand photograph facing No. 2, was 

 taken with the Astrographic telescope of thirteen inches 

 aperture, used for the International Survey of the Heavens; 

 the exposure, as for its companion picture, was eight hours. 

 Here, again, we have a picture strongly contrasted with 

 the two we hive already considered. Like the second, it 

 has been enlarged from the original negative, and is on a 

 scale nearly four times as great as No. 2, and about twelve 

 times as great as No. 1. The contrast between it and its 

 companion picture is very great, and a hasty glance might 

 lead one to give the palm of superiority to the picture on 

 the smaller scale ; it is certainly the more impressive. 

 The moment, however, that we carefully examine the 

 photographs, we find that No. 3 shows us many more stars 

 than No. 2 ; that these stars have discs both relatively and 

 absolutely much smaller than the corresponding stars on 

 the other plate ; and that what in No. 2 is a mere 

 " smudge " of nebular light, is here presented to us with 

 a refined delicacy of detail. We trace at once an associa- 

 tion of stellar points with the intricate configurations of 

 the nebula, but it can scarcely escape us that some of the 

 stellar groupings are perfectly irrespective of the nebula. 



On the other hand, the enlarged scale causes us to lose 

 something. The area covered is but one-seventieth of 

 that of No. 1 and but one-twelfth of that of No. 2, and the 

 consequence is that the nebula and the star clusters are 

 not thrown up into such distinctness by contrast with the 

 darker regions surrounding them. Vast areas of nebulosities, 

 too, that come out bright and definite in the second plate, 

 are here entirely unrepresented, or merely suggested and 

 hinted at, at the best. The "personality" of the two 

 instruments is brought into striking evidence ; the one 

 revealing much more definitely the extent of the difiusion of 

 the nebulous matter, the other far surpassing it in the 

 amount of deUcate detail it reveals, and in its suitability 

 for yielding precise determinations of star places under 

 the micrometer. 



Our fourth plate is of an altogether different order ; it 

 is Sir John Herschel's drawing of the nebula, as made 

 with his twenty-foot reflector. Here we are at once struck 

 with the inferiority of even the most careful and painstaking 

 drawings to the results obtained by photography with much 

 smaller instruments. To begin with, great stretches of 

 nebulosity clearly revealed to the photograph have escaped 



even the practised vision of Sir John Herschel. Further, 

 the field opened to him at any single view was far more 

 restricted than can easily be commanded by a single 

 photographic plate. But, above all, the complexity of the 

 object made its delineation " a work of great labour and 

 ditficulty," and occupied him for many months of the 

 most exacting toil, until, as he himself said, he almost 

 despaired "of ever being able to transfer to paper with 

 even tolerable correctness its endless details." 



There is yet another difficulty to which our attention is 

 at once drawn when we compare Herschel's sketch with 

 the photographs. One of the most striking features of the 

 sketch is a bright zig-zag, almost exactly on the central 

 horizontal line of the sketch, but a little to the left of the 

 centre. In shape it is rather like a capital S that has 

 been slightly smudged. Mr. Ranyard spoke of it as a 

 curious trident-shaped structure ; Dr. Gill compared it to 

 a swan. In the sketch it is inferior in brilliancy only to 

 the part of the nebula immediately surrounding the " key- 

 hole." In the photographs which we have yet examined 

 it is non-existent. At first sight, therefore, we have here 

 most definite evidence of change in the nebula. But we 

 must bear in mind that whilst we can compare the bright- 

 ness of two objects, both within our sight at the same 

 time, it is most difScult to carry such a comparison in our 

 minds, and more difiicult still to express it accurately in a 

 sketch. 



The fifth plate gives us a finer result still. Here we 

 have the Astrographic telescope again at work, and the 

 eight hours' exposure being now lengthened out to twelve, 

 we have not only the sharp stellar images, the numerous 

 stars, and the delicate detail and large scale of No. 3, but 

 we have also an extent of nebulosity very nearly equal to 

 that of No. 2. 



The sixth and seventh plates need not detain us. Their 

 principal value was well pointed out by Mr. Ranyard at 

 the time, in the striking evidence which they afford of the 

 radiation of star streams from the nebular centre. 



We now come to the photograph with the Bruce tele- 

 scope. The scale, it will be noted, is precisely that of the 

 sixth plate ; the exposures two hundred and forty minutes 

 as compared with three hundred and forty-three ; but the 

 efl'ective light-gathering power of the telescope was not 

 far short of four times as much. Comparing it with the 

 wonderful photograph of Dr. Gill (No. 5), with an exposure 

 three times as great, and therefore practically not differing 

 much in total light reception from No. 8, we find that the 

 later plate is distinctly inferior as regards the number of 

 stars shown, but in extent of nebulosity it has a distinct 

 superiority, and is very strictly comparable in this respect 

 with No. 2, to which double the exposure had been given. 

 The ratio of aperture to focal length for No. 2 and No. 8 

 is practically the same. The result, therefore, of con- 

 structing a telescope of four times the linear dimensions of 

 that employed for No. 2 has been to secure the same 

 extent of nebulosity with half the exposure, to render it 

 on the original negative on a scale four times as great, 

 and by giving to the stars much smaller discs to exhibit 

 the nebula more nearly free from stellar blurring. 



Careful examination will bring out many interesting 

 features. I think the following are all that need be noted 

 now : — 



First, the swan-neck of Herschel's drawing, which is 

 just hinted at in the Gill photograph (No. 5), is here very 

 feebly but unmistakably visible. Tiie change, therefore, if 

 change there has been in the nebula since the days of 

 Herschel's observation, has been probably one, not of shape 

 but of relative brilliancy ; and this, I think, is easier for 

 ua to understand. 



