August 1, 1892.] 



KNOWLEDGE 



149 



Holden's scrutiny of the " Saturn planetary '' in August, 

 1888," with the Lick 30-inch, deserve careful considera- 

 tion. He found it to consist mainly of a star, or nebulous 

 nucleus, surrounded by an elliptical ring lying upon 

 an oval shield much less %-ividly luminous, while two 

 exterior nebulous patches, situated nearly in the pro- 

 longation of the major axis of the ellipse, seemed to be 

 connected with it by faint gleams of phosphorescent illumi- 

 nation. These represented the " ans;e " of earlier 

 observations, and indeed their detachment from the main 

 body had been suspected at Parsonstowu in 1852, and 

 pretty well made out in 1862. Possibly, they are embryo- 

 satellites of the nebulous system they are still partially 

 attached to ; nor can their position relative to its longest 

 diameter be easily regarded as accidental. We are irre- 

 sistibly led, on the contrary, to trace an analogy between 

 them and the nebulous efl'usions from the extremities of the 

 major axis of the annular nebula in Lyra , and to infer in 

 both cases the genuinely oval shape of the objects presented 

 to our view. For why should a mere perspective effect be 

 emphasized in any way by physical configuration ? 



The nebula in Aquarius was perceived by Professor 

 Holden as of a pale blue, but its stellar nucleus as white ; 

 the difference of colour being in fact so decided as to require 

 a change of focal adjustment in passing fi'om one to the 

 other. The interior arrangements of the nebula were 

 evidently extremely intricate. The central oval, instead of 

 being bounded by a smooth curve, " looked like an elastic 

 link which had been warped." A sudden failure of light 

 in the glimmering ring near the southern extremity of its 

 minor axis enhanced the effect of distortion ; but a helical 

 form, though suggested, could not clearly be made out. 



A strikingly similar object is situated in the constellation 

 Andromeda (N.G.C. 7662). Lnperfectly seen at first as a 

 unifonn. greenish-blue disc, an interior vacuity detected at 

 Parsonstown betrayed its true nature to be rather annular 

 than simply planetary. Nor is the ring it includes by any 

 means symmetrically shaped. Lassell considered it to be 

 bi-annular ; Professor Yogel was impressed with the warped 

 and twisted aspect of what may conceivably prove to be a 

 multiple combination of rings thrown off in various planes. 

 Closely-wound spiral branches, and a central star, were 

 perceived with the Eosse reflector ; but the object appeared 

 starless to Dr. Struve in 1847, as well as to Searle. using 

 the Harvard 15 -inch refractor, in 1866. Lassell gave 

 32" by 28" as the dimensions of the outer ellipse ; and saw 

 the " central star " in the guise of a minute planetary disc, 

 tinged with blue. To Vogel, however, it was entirely 

 in\asible, in spite of his best efforts for its discovery. Yet 

 he noticed the sparkling appearance which had misled 

 Father Secchi into the belief that he held in the field of 

 his equatorial a " magnificent ring of stars." f For the 

 " horse-shoe of star-dust " in Andromeda, like its niter eijn 

 in Aquarius, gives the usual gaseous spectrum of planetaries, 

 which is now known to mclude, as a fourth visible line, 

 the blue ray of wave-length 469, distinguishing stars of 

 the ^^'olf-Rayet type. 



The photographic study of these two nebuhe, lately set 

 on foot by Dr. Scheiner at Potsdam, may be expected to 

 add much and rapidly to our knowledge of their nature 

 and conformation. The images obtained of them, although 

 only half a millimetre in diameter, show a considerable 

 amount of detail. They confirm the annular shape attri- 

 buted to them on the warrant of telescopic observations, 

 and bring out, with singular strength, the central nuclei 

 which the best telescopes have not always availed to 



Monthly Notices, vol. xlviii., p. 391. 

 t Asfr. Xack.. Xo. 1018. 



display. In the photographs these are, nevertheless, the 

 brightest joarts of each formation. Yet they are mere 

 irregular condensations, with no pretentions to a stellar 

 nature. The superiority of their actinic power repeats the 

 phenomenon first brought into notice by photographs of 

 the Lyra nebula, and seems to point to a general law. 

 Dr. Scheiner thinks it can only be accounted for by sup- 

 posing a predominant quantity of some peculiar gas 

 emitting, in the main, highly refrangible light, to be 

 collected in the central regions of planetary nebulae -,1 yet 

 the resulting nuclei, when they can be seen at all, shine 

 with a white light, bear a star-like aspect, and probably 

 give continuous spectra. The problem of their real con- 

 stitution is thus far from easy to solve. But, whatever the 

 secret of their photographic effectiveness, it is already 

 tolerably evident that they play a part of fundamental 

 importance in determining the structure of planetary 

 nebulae. They are perhaps the primary seats of the forces 

 by which these interesting objects are moulded into 

 characteristic shapes ; and the circumstance may be 

 regarded as a particularly fortunate one that the camera is 

 so well adapted to display and emphasize their complex 

 relationships with the nebulous masses organized, as it 

 were, under their immediate control. 



M' 



LIGHTNING PHOTOGRAPHS. 



By A. C. Ranyard. 

 Pi. GLEW, whose lightning photograph appears 

 at the top left-hand corner of our plate, has 

 succeeded in roughly measuring the duration 

 of a flash of lightning. The ingenious 

 method he adopted will be foimd described 

 in his letter published in our correspondence column. 

 There is some doubt as to the direction in which the lens 

 with which the photograph was taken was moving during 

 the interval between the flashes. If, as seems most 

 probable, the lens was moving at the time of the flash 

 from the left-hand side of the plate towards the right, 

 the first flash must have been the faintest, and it must 

 have died away comparatively slowly to be succeeded after 

 an interval, which probably was not greater than about 

 the fortieth of a second, by a brighter flash, the duration 

 of which must have been less than the two-hundredth of 

 a second. Again, after a sUghtly longer interval (perhaps 

 the thirtieth of a second), there came the brightest of 

 the three flashes, which died away in an interval less than 

 the two-hundredth of a second. 



If instead of moving from left to right the lens witli 

 which the photograph was taken was moving from right to 

 left during the interval between the flashes, the first flash 

 must have been the brightest, and it must have begun 

 with a faint glow which brightened gradually during an 

 interval less than the two-hundredth of a second until it 

 became intensely brilliant, and then suddenly ceased. 

 The scale of our plate is not sufficient to show this, but 

 the shading oft" of each of the photographic images of 

 the flashes towards the right hand is distinctly recognizable 

 in the silver prints from Mr. Glew's negative. Probably 

 the duration of the discharge, and its mode of ai-ri%-ing at 

 any particular point in its course, varies with the con- 

 ducting power of the air or cloud or the terrestrial bodies 

 from which it is derived. This is well illustrated by the 

 old experiment of placing a httle loose gunpowder over 

 the interval between two wires, and dispersing it without 

 explosion by the discharge from a Leyden jar sent through 

 the wires, after which a few inches of wet string are inter- 



it: Asfr. ync/i,, 'So. .30b6. 



