August 1, 1889.] 



♦ KNOWLEDGE ♦ 



211 



i-eproduce the phenomenon in photographs of electric 

 spaiks. 



The first experiment was to photograph a h\rge number 

 of sparks in a dark room on the same plate, but although 

 the images crossed and recrossed each other in all directions 

 there was no indication of reversal. Each spark gave a 

 clear image standing out boldly on the dark background, 

 and points of crossing were even extra bright. 



The second was to allow the images of a series of bright 

 sparks from large Leyden jars to impress themselves across the 

 ])late. It was then removed from the camera and exposed to 

 dillused gaslight, so that different portions were acted upon 

 for difl'erent lengths of time. On developing it was seen 

 (fig. 1)* that the images of the sparks were bright, with a 



daik margin (I speak always of the print obtained), and 

 that this dark margin spread inwards towards the centime as 

 the duration of the exposure to gaslight increased. 



Next, I took a series of somewhat less brillant sparks, and 

 exposed half the plate to gaslight for a second. The sparks 

 came out bright on the part not exposed to gaslight, and 

 reversed on the other (fig. 2). It was thus evident that a 



reversal was easily produced by subsequent general ex- 

 posure, but the next point was to suggest how that 

 subsequent effect could be produced in nature. 



Of course my first experiment did not correctly represent 

 what occurs during a thunderstorm. My sparks were pro- 

 jected upon a dark background. Lightning flashes have 

 masses of white clouds behind them. Hence I photographed 

 a number of sparks, after having previously placed a sheet 

 of white card behind the terminals in order to imitate the 

 cloud. Some of the earlier sparks showed reversal, and 

 and others exhibited marginal reversal with the white core 

 already remarked upon (fig. 3). 



This experiment seemed almost coucliisivo, and, after 

 repeating the whole series to confirm the results, I felt 



• The woodcuts illustratiug this letter have beou kindly lent by 

 the Editor of the Eleutru-ian, in which a short report of Jlr. 

 Clayden's paper read before the Physical Society was printed. — 

 Editor. 



assured that I had found the clue to the mystery of dark 



Since then I have pursued the subject further through a 

 long series of experiments which I propose to publish in 

 due time. INIeanwhile I may say that I have abundant 

 evidence to show that the revei'sal of the images is due to 

 subsequent exposure to diffused light, which is apparently 

 powerless to reverse subsequent images. I have therefore 

 had no difficulty whatever in imitating all the observed 

 phenomena, such as the crossing of a dark image by bright ones. 

 From a meteorological point of view this is sufficient. Dark 

 flashes have no existence in nature. They ar3 not even 

 due to any peculiarity of lightning discharges, such as 

 an oscillatory series of flashes. They are a purely photo- 

 graphic efl'ect, the explanation of which belongs to the most 

 obscure portion of the physics of photography. I do not 

 attempt even to suggest a theory at present, but I hope that 

 my experiments, some of which have been kindly suggested 

 to me by Sir- G. G. Stokes, wOl at any i-ate provide a good 

 deal of material for future elucidation. 



In conclusion, allow me to apologise for trespassing so 

 largely on youi' valuable space. — ^ Yours very truly, 



Arthub W. Clayden. 



Warleigh, Palace Road, S.W. : 

 July 19, 1889. 



PHOTOGIUI'H.S OF THE OKION NEBULA. 

 To the Editor of Knowledge. 



Dear Sir, — 1 have examined Professor Pickering's 

 lantern slide of the Orion Nebula referred to in his letter in 

 the July number of Knowledge. I infer that this slide 

 represents some of his best work. If so, I cannot trace any 

 nebulosity connecting together the three nebuhe (M 42, 

 M iS, and h 1180) upon it. There is more shown upon my 

 early photogi-aph of November 188G than can be seen upon 

 this slide, but there is a noticeable light patch to the north 

 following of M -13, which looks like, and has po.ssibly been, 

 assumed by Professor Pickering to be a nebulosity joining 

 the two nebulw. A comparison with my negatives, how- 

 ever, shows that it is really a photogra[)hic defect or stain 

 on the film. That this is the case is proved by the fact that 

 I can only count twenty-two stars on Professor Pickering's 

 photograph within a circle which is described so as to 

 contain the nearest margins of M 43 and h 1180, and 

 includes the area of the connecting nebulosity, while ou my 

 photograph I counted within a similar ai-ea (taking into 

 account of course the difference of scale) fiftj'-four stars. 

 I am unable to judge what may be seen on Professor 

 Pickering's original negative ; but, on the other hand, he 

 cannot judge of what can be seen on mine from the enlarge- 

 ments published in Knowledge. All negatives lose greatly 

 by copying. I await with interest the publication of 

 Professor Pickering's promised paper. — Yours faithfully, 



Isaac Roberts. 



Kennessee Maghull, near Liverpool : 

 July 11, 1889. 



To the Editor of Knowledge. 



Sir, — Some twenty years ago I noticed a curious phe- 

 nomenon in the case of a large family with which I was 

 intimately acquainted, namely, that the eldest chUd i-e- 

 sembled closely iu appe;iranco, manner, and disposition the 

 maternal parent ; the second eldest bore the sjvmi- close 

 resemblance to the father ; the thii'd, again, i-esembled the 

 mother, and so on through the whole family. It struck me 



