I04 



NA TURE 



[November 30, 1899 



For perfection of simplicity some method, whether it be 

 graphic or tabular, for converting observed numbers into per- 

 centiles, might be printed at the back of the blank form. 



Francis Galton. 



On the Cause of Dark Lightning and the Clayden 

 Effect. 



I HAVE been criticised in a letter which appeared recently in 

 Nature for not alluding in my letter on dark lightning to the 

 peculiar photographic reversal known as the Clayden effect. I 

 must confess that at the time of writing my letter I was unaware 

 of this effect, a description of which has only appeared, so far 

 as I know, in one of the photographic journals. Mr. Clayden 

 has certainly explained dark lightning, and it only remains to 

 explain his explanation. As I think that this effect is not 

 generally known, I believe that it may be worth while to de- 

 vote a few words to the statement of the case before describing 

 the experimental work by which I have determined the factors 

 which play a part in this very curious photographic phenomenon. 



Mr. Clayden showed that if a plate which had received an 



Fig. I shows a series of spark images, some normal, some 

 partly reversed, and others wholly reversed. The sparks are 

 those of a large inductorium with a good-sized Leyden jar in 

 circuit. The sparks were all of equal intensity, but after each 

 discharge the iris diaphragm of the lens was closed a little. 

 It will be seen that the borders of the bright sparks are re- 

 versed. In some the image is reversed, with the exception of 

 a narrow thread down the core. The images were impressed 

 in succession on the plate by moving it in the camera. A plate 

 holder was dispensed with, an opening being made in the 

 ground-glass back by removing a strip a few centimetres wide. 

 The plate was held against this opening, and a large number 

 of exposures made in a few moments. Of course, the room 

 was in total darkness. After exposure, the plate was exposed 

 to the light of a candle for a second or two, and then developed. 



In this series of pictures it will be seen that the edges of the 

 bright images of the sparks are reversed, the intensity on the 

 border of the image being less than at the core. As the inten- 

 sity of the spark becomes less and less, the bright central core 

 dwindles down to a mere thread, and eventually disappears, 

 the spark's image being feeble enough to reverse over its entire 

 area. 



This explains why the dark lightning flashes are usually 



Fig. 



impression of a lightning flash or electric spark was subsequently 

 slightly fogged, either by exposing it to diffused light or by 

 leaving the lens of the camera open, the flash on development 

 came out darker than the background. If, however, the plate 

 was fogged before the image of the flash was impressed, it came 

 out brighter than the background, as in the ordinary pictures 

 of lightning. I refer to the appearance in the positive print 

 in each case. This is quite different from ordinary reversal 

 due to the action of a very intense light, for the order in which 

 the lights are applied is a factor, and the phenomenon lies 

 wholly in the region of under-exposure. I repeated Mr. Clay- 

 den's experiment, and obtained dark flashes without any 

 difficulty. 



The effect cannot, however, be obtained by impressing an 

 image of the filament of an incandescent lamp on a plate, and 

 subsequently fogging the plate. Clearly there is something 

 about the light of the electric spark which is essential to the 

 production of the reversal. It is not intensity, however, for I 

 found that it was impossible to obtain reversed images of bright 

 sparks with the lens wide open. 



NO. 1570, VOL. 61] 



ramifications of the main flash. The ramifications are less * 

 brilliant discharges and reverse, while the main one is too bright j 

 to cause the effect. _ \ 



The first thing that occurs to one is that it may be some ; 

 peculiar radiation, which the spark emits, which is wanting in the ! 

 light coming from other bodies. If a small photographic plate 

 is partly screened by a piece of black paper and illuminated by , 

 the light of a small spark at a distance of two or three feet, and • 

 a similar plate, screened in the same manner, is illuminated for< 

 a moment by candle light of sufficient intensity to produce the,, 

 same amount of blackening on development, we shall have the^' 

 means of showing that the spark light differs in its action on the' 

 plate from that of the candle. If these two plates, before; 

 development, be half-screened in a direction at right angles tO; 

 the former one, and exposed to the light of the candle for a; 

 second or two, the part of the plate which has been illumin-j 

 ated by spark light plus candle light does not become asj 

 black on developing as the part which has received candlej 

 light alone, whereas the part which has been twice exposed- 

 to candle light is blacker than that which has been only ex-" 

 posed once. This shows that the light of the spark does not 

 act in the same way as the light of the candle. WhereinJ 



