218 



KNOWLEDGE. 



[September 1, 1897. 



detail movements, ju3t as M. Marey has already doue with 

 the flight of insects. In the same way, of course, slow 

 movements may be kinetos;raphed at leisurely speed, and 

 projected much more quickly. By this means it is possible 

 to observe in a few minutes, in the most wonderful manner, 

 the whole growth and development of a plant, from its 

 first tiny shoot above the earth to the completion of 

 flowering, fruiting, and the fall of the leaf ; or, in the 

 same way, a record could be made of a man's development 

 and decline — " from the cradle to the grave " — the whole of 

 which could be represented in a few minutes. But I must 

 not speculate on the possibilities of the process. My wish 

 has been to make it clear that kinetographing is far from 

 being merely the passing fancy of the music-hall public ; 

 that it is a new and powerful means of scientific record 

 and research. 



PHOTOGRAPH OF THE NEBULA HERSCHEL 

 V 14 CYGNI. 



By Isaac Roberts, D.Sc, F.R.S. 



THE photograph annexed covers the region between 

 E.A. 20h. 49m. 4s. and R.A. 20h. 54m. 493. 

 Declination between 30° 14-2' and 31° 506' north. 

 Epoch 1900. 

 Scale, one millimetre to twenty-four seconds of arc. 



The star precedinii the nebula and nearly opposite the 

 centre is D.M. 4264 Zone + 31°, magnitude 6-7, R.A. 

 20h. 49m. 24-7s., Declination -f 31° 15-0'; and the star 

 fohouinq the nebula is D.M. 4292 Zone + 31°, magnitude 

 7, R.A."20h. 54m. 33-7s., Declination -f 31° 154'. 



The photograph was taken with the twenty-inch reflector 

 on November 4th, 1896, between sidereal time 21h. 18m. 

 and Oh. 13m., with an exposure of the plate during 

 2h. 55m. 



The nebula is No. 6992 in the New General Catalogue, 

 and has been observed and described by Sir J. Herschel 

 and Lord Rosse ; but only photographs can present it in a 

 form that is intelligible, for it is not possible to depict by 

 hand, with sufficient accuracy, the complex structure of 

 this and of similar nebuht. 



On examination of the photograph we see that the 

 nebula extends in suuth foUou-imj to north jirecedimj direc- 

 tions, and measures eighty minutes of arc in length ; its 

 structure is wavelike, with indications of irregular dis- \ 

 turbances throughout its mass, and at the southern part 

 there is a half-drum-like formation of the wave structure, 

 with extensions of the nebulosity towards the south. The 

 wave structure has immersed in it faint nebulous stars as 

 well as irregular condensations of nebulous matter, whilst 

 stars of the normally finished form, between the tenth and 

 seventeenth magnitudes, are densely strewn over the whole 

 surface of the nebula and over many degrees of the sky 

 around it. 



A photograph measuring seventeen degrees in diameter, 

 with the nebula as the centre, was taken simultaneously with 

 the reproduced plate, but with the Cooke five-iuoh lens. This 

 shows that there are patches of this wavelike nebulosity 

 extending over an area of two and a half degrees in diameter 

 in the direction prccedivji, and it includes the nebula i 

 Herschel V 15 Cygui (N.G.C. 6960). The plate also shows, 

 very prominently, large areas of cloudiness with reaches of 

 clear areas dividing them, which, by the efi'ect of contrast, ! 

 have led many to assume that they are areas of true nebu- 

 losity, but in reality they are due to the light of countless 

 numbers of faint stars that on the small scale of a portrait- 

 lens photograph appear to be closely crowded together and 

 resemble nebulous areas. To the eyes of the experienced 



examiner the true nebulosity can with certainty be distin- 

 guished from the star-cloud effects here referred to. 



It is not desirable at this early period— the commence- 

 ment of photo-astronomy — to speali with the confidence of 

 demonstration concerning the origin of these wonderful 

 objects — the nebuhi' and the appearances as of whirlpool 

 eddies amongst the stars all over the Milky Way ; but our 

 aim should be to dispel the mysterious by investigations, 

 fitting together links in the chain of evidence, notwith- 

 standing that we may insert some which will have to be 

 replaced later on by others. 



Attention has already been drawn, by aid of photographic 

 illustrations in previous numbers of Knowledge, to the 

 similarity of the curves of the stars involved in spiral 

 nebulfe to the curves and aggregations of stars shown in 

 the Milky Way, and that there is apparently a connection 

 between them of cause and efi^ect. But the nebula shown 

 on the annexed photograph is of a different character, and 

 the absence of gravitational effects, excepting the indication 

 at the southern end, is conspicuous. The general appear- 

 ance of the nebula is that of precipitation of invisible 

 matter — either gaseous or of dust particles —which exists 

 in space as clouds of vast extent. 



That the methods of stellar evolution are various we 

 have ample evidence in the different forms of nebulas which 

 have been photographed. There are planetary, annular, 

 elliptic, spiral, globular, and unsymmetrical nebula: ; and 

 they are presented to us at various angles in projection 

 between edgewise and full face. Like the rings of Saturn, 

 their superficies are enormously greater than their thick- 

 ness ; and it is quite conceivable that these objects may 

 have been formed out of the invisible clouds of material to 

 which I have alluded. 



We know of no body whatever existing in space which 

 has no motion of translation ; but whether this invisible 

 cloudy matter is in motion or at rest, it could be run into 

 by another body that is in motion, with the result that 

 whirlpool motions would be set up that would eventually 

 develop into nebulfe of various forms, such as those which 

 have already been, by photography, shown to exist. 



If, on the other hand, the clouds themselves are in 

 motion and collide with each other, then vortical motion 

 would be set up over large areas, giving rise by progress 

 of development to such nebul* as are represented by the 

 annexed photograph. This nebula shows signs of fission, 

 and may pass in its process of development into sym- 

 metrical nebulae and into stars, and again from stars 

 into — what ? 



ASTRONOMICAL PHOTOGRAPHY.-IL 



The Conditions which Determine the Limiting Time of Exposure. - 



By F. L. 0. Wads WORTH, 

 Astroijliysicist, Yerkes Observatory. 



CASE OF STARS. 



THUS far we have been considering only the case 

 of nebulfc and other extended sources. In the 

 case of stars the conditions are very different. 

 The intensity of a star image varies directly 

 as the fourth power of the aperture, and in- 

 versely as the square of the focal length and the square 

 of the mean (effective) wave-length. Neglecting this 

 last factor, which is of secondary importance in the 

 present considerations, we may also say that the intensity 

 varies as the square of the angular aperture multiplied by 

 the square of the linear aperture. Putting it in ttiis way 

 we see that since the brightness of the field also varies as 

 the square of the linear aperture, the only way of 



