Auoi-sT. 1901. 



KNOWLEDGE 



181 



involved in it. The brighter stars niiiy not bo involved 

 in the nebula but seen in alignment with it. 



There is no indication that the nebula has great di>pth, 

 but rather that it resembles a relatively thin cloud which 

 can p.utlv 1)0 seen through, and the" two bright stars 

 apparently involved in it have projections of nebulous 

 matter extending from them which somewhat resemble the 

 projection from the star Ehctra in the PJ,uuI,g. In order 

 to see these appearances clearly it is necessary to examine 

 the origiuiil negative, for they are too faint to be reproduced 

 ou paper prints. 



NEBULA Ijl IV. 2 MONOCEROTIS. 

 R.A. 6h. 33m. 42s. Decl. 8° 49'-3 North. Epoch 1900. 

 Scale — one millimetre to twelve seconds of arc. 



References. 



X.G.C. 2261. G.C. 1437. h 399. y IV. 2. Rosse, 

 Ohs. of Neb. and CI. of Staff, p. .53. Phil. Trans.. 1833, 

 PI. XIV., Fii,'. t)4, and 1850, PI. XXXVII. Pig. 10. 

 Lassell, Mem. li.A.S., Vol. XXIII., PL IL, Pig. 8. 



The photograph was taken with the 20-inch reflector on 

 January 2~tb, 1900, between sidereal time 3h. 43m. and 

 5h. 13m., with an exposure of the plate during uiuety 

 minutes, and it shows the nebula to be in form like a fan, 

 or comet, with a star of about 11th magnitude at the apex. 

 There is faint nebulosity with structural characteristics ou 

 the north preceding side, and an extension, like a streamer, 

 on the north followimj side. The nebula resembles the 

 two that are near Gamma Ca^.'siopeia (Y>nh. in Knowledge, 

 Mav, 1898; and in I.B Pho. ofNeb.and Cl.of Sfars,Yo\. II., 

 PI.' XXV.). It also reseuibles the nebula IJ. I. 143 

 Virginig. Two or three condensations are visilile in the 

 nebulosity. In the Phil. Trans., 1850, PI. XXXVII, 

 Fig. lO, Lord Rosse has given a drawing of this nebula 

 on which is shown a ring separating the apex from the 

 body, but the photogi'aph does not indicate that 

 appearance. 



NEBULA ^ IV. 28 CORVI. 

 R.A. llh. 56m. 46s. Decl. 18° 19'-2 South. Epoch 1900. 

 Scale — one millimetre to twelve seconds of arc. 



References. 

 N.G.C. 4038-9. G.C. 2670-1. h 1052-3. Ip IV. 28. 



In these references the nebula is described as two 

 nebulae, but Lord Rosse, in the Phil. Trans., part 3, 1861, 

 p. 724, PI. XXVII.. Fig. 18. depicts and describes it as a 

 spiral which in outline corresponds with the photograph 

 hereto annexed. 



The photograph was taken with the 20-inch reflector on 

 April 25th, 1900, between sidereal time llh. 42m. and 

 13h. I2m., with an exposure of the plate during ninety 

 minutes, and it shows the nebula to be a right-hand 

 spiral with fourteen or fifteen star-like condensations in- 

 volved in the convolutions, one of which (or probably two 

 or three images overlapping), forms the nucleus, and there 

 is shown on the negative extensions of faint nebulosity 

 in the north preceding, south, and south folloioiiuj directions. 

 There are some deviations from the symmetrical form on 

 the north preceding side, both in the convolution and in 

 the star-like condeusations involved in it, and ere long, 

 measurements of the position angles, distances, and 

 fiducial places of reference between the condensations and 

 some of the normal stars surrounding the nebula will be 

 published. Then by aid of the photographs, of diagrams, 

 and of tables of measurements from such fiducial stars, 

 the astronomers of the future will be well equipped for 

 Sliding out the changes and the progressive development of 



this and of other spiral iiebuliB into star-clusters, or else 

 into systems of stars similar to the innumerable curves and 

 lines of stars which photographs have revealed to us as 

 large constituents of the system of the Milkij Way. I have 

 now many photographs of spiral nebu!;e ready for treat- 

 luent in the manner hero indicated, and have no hesitati(m 

 in saying that the most promising field for astronomical 

 investigations in the present and in the immediate future 

 will be that in connection with the spiral nebulie, the 

 result of which will be demonstration of the evolution of 

 stellar systems in this manner from roaming clouds of 

 nebulous or meteoric matter in the expanse of space. 



NEBULA MESSIER 61 VIRGINIS. 

 R.A. 12h. 16m. 48s. Decl. 5° 1'7 North. Epoch 190O. 

 Scale — one millimetre to twelve seconds of arc. 



References. 



N.G.C. 4303. G.C. 2878. h 1202. Ijl I. 139. Rosse, 

 Ohs. of Neh. and CI. St., p. 113. Phil. Trans., 1833. 

 PI. XV., Fig. (■,9, and 1861, PI. XXVII., Fig. 21. 



The photogra])h was taken with the 20-inch reflector on 

 May 7tli, 1899. between sidereal time I2h. 22m. and 

 14h. 25m., with an exposure of the plate during two hours 

 and three minutes, and it shows the nebula to be a right- 

 hand spiral viewed in a direction nearly iierjiendicular 

 with its plane. It has a bright stellar nucleus, and 

 several faint condensations in the convolutions, and two 

 stars in the nebulous spaces between them ; there are also 

 two prominent tangential projections of nebulous matter 

 ou the north following side with a star at each termination, 

 and they suggest the idea of projection by centrifugal 

 force. Similar appearances are also seen on the ]ihoto- 

 graphs of the Great Spiral Nebula M. 51 Canum (pub- 

 lished in I.R. Photos, of Neb and CI. <•/ Stars, Vol. II., 

 PL XV.). There is also shown ou the photograph hereto 

 annexed a small faint spiral nebula with a stellar nucleus 

 and three or four condensations in the nebulosity, thirty- 

 two seconds following and 5'9 north of M. 61, which is not 

 recorded in the catalogues, and therefore may not have 

 been observed before. 



THE SECOND SERIES OF LINES IN THE 

 SPECTRUM OF HYDROGEN.' 



The presence of a second series of hydrogen lines, in 

 addition to the ordinary series, in the spectrum of ^ 

 Puppis, was announced in Circulars Nos. 12, 16, and 18. 

 Acciu'ate wave lengths could not then be determined 

 for the less refrangible lines. Since then, measures have 

 been made of six photographs of spectra of K Puppis, and 

 two of spectra of S Orionis. Following the notation proposetl 

 by Vogcl for the ordinary series of hydrogen lines, the 

 new series may be designated Ha', H/3', lly', etc. A 

 comparison of the wave lengths recently determined with 

 two computed values, is given iu the following table. 

 The first of these is given in Circular No. 16, and is a 

 slight modifieatiou of Balmer's formula. It is 3646.1 

 nr^^, in whicli n is an even number for the ordi- 

 nary series of hydrogen lines, and an odd number for 

 the additional series. The second formula is 

 ^ = A + B -, + C -,. This form was proposed 

 by Kayser, immediately after the announcement of the 



• Abridged from " Uarvai-d College Observatory Circular," No. .'JS. 



