June 12, 1890J 



NATURE 



153 



slowly instead of quickly, I think it is clear that the 

 appearance of one or more ribbon-like flashes, like those 

 in Fig. 2, would have been produced. 



But the photograph of a flash may possibly assume a 

 distinctly broadened form, perhaps more suggestive of a 

 flattened wire than of a ribbon, when the camera is 

 absolutely steady. In such cases it will generally (per- 

 haps always) be seen that one edge of the imsge is sharp 

 and clear, while the other is ill-defined and hazy. I have 

 succeeded in imitating this eftect very well in photo- 

 graphs of the machine spark : it is obtained when the 

 light does not fall perpendicularly, or nearly so, upon the 

 sensitive plate, and is no doubt due to successive reflections 

 between the surfaces of the lens. [Exhibited.] 



Lastly, we have to consider the so-called " dark 

 flash." It occasionally happens that, on developing a 

 photographic plate which has been exposed during a 

 thunderstorm, the image of a lightning flash comes out 

 black instead of white. Fig. i presents a striking in- 

 stance of this phenomenon. Black ramifications are 

 seen to proceed outwards on both sides of the main 

 bright flash ; there is also what appears to be an in- 

 dependent black flash which starts from the top of the 

 picture and crosses the bright one near the knot. The 

 origin of this strange appearance was for a long time a 

 mystery. No one had ever seen a dark flash with the 

 unassisted eye, and the question arose, whether the dark 

 images in the photographs really represented a hitherto 

 unobserved physical effect which occurred in the air 

 itself, or whether, owing to some optical or chemical 

 action taking place inside the camera or upon the sensi- 

 tive plate, the impression of a luminous flash became 

 converted into a dark one. There is no need to discuss 

 the several ingenious hypotheses which were suggested 

 in explanation of the anomaly ; it is sufficient to say that 

 the mystery was completely cleared up a few months ago 

 by the experiments of Mr. Clayden. The fact, as de- 

 monstrated by him, is shortly this. If the lens of the 

 camera be covered the moment after a flash has occurred, 

 the developed image will always come out bright, feebly 

 or strongly according to circumstances. If, however, 

 the plate be exposed after a flash has acted upon it, 

 either to the continued action of a feeble diff'used light or 

 to the powerful glare arising from one or more subsequent 

 flashes, then on development the image of the original 

 flash will probably come out black. The effect is there- 

 fore not a meteorological or physical one, but purely 

 chemical. It can be obtained not only with a lightning 

 flash, but also with a machine spark, or even with an 

 ordinary flame. It is merely necessary that the plate 

 should be exposed to the action of a certain amount of 

 light after it has received the impression and before 

 development. 



Some photographs which I have made of machine 

 sparks fully confirm this explanation of Mr. Clayden's. 

 The room was illuminated by a single gas-jet, and the 

 background was a white screen with a black post in the 

 middle of it (see Fig. 3). Two series of sparks were 

 passed between the ball terminals of an electrical machine 

 and photographed. After the first series were taken, the 

 lens was left uncovered for half a minute ; then it was 

 capped, the camera shifted slightly, and the second series 

 taken ; the lens was again left open for half a minute, 

 and the plate afterwards removed from the camera and 

 developed. It will be seen that while the second series 

 of sparks come out bright in the natural way, the first 

 series have been reversed and blackened by the action 

 for one minute of the light reflected from the white screen 

 upon the undeveloped image. Exposure to the diffused 

 light for half a minute only was not in this case sufficient 

 to cause reversal. 



These experimental results make it almost certain that 

 the flash in Fig. l was really a double one. The first 

 flash was comparatively feeble, and possessed the lateral 



NO. 1076, VOL. 42] 



ramifications characteristic of an incomplete discharge. 

 The second, which probably occurred immediately after- 

 wards, was a powerful one without ramifications, and 

 followed accurately the main path traced out by the 

 other. The glare arising from this second discharge 

 caused the photographic reversal of the ramifications 

 belonging to the first. 



Everyone must have noticed the proverbial quiver of a 

 lightning flash. This peculiar effect is often due to the 

 multiple discharge of which we have already spoken. 

 Sometimes, however, I believe the phenomenon is a 

 purely subjective one, depending upon a certain physio- 

 logical reaction of the optic nerve. If we gaze at a bright 

 flame which is suddenly uncovered and immediately ex- 

 tinguished, then after a very short interval of darkness a 

 distinct but transient image of the flame will reappear ; 

 and it is even possible that after another brief interval a 

 second after-image of the flame may be seen. It is, 

 however, by no means easy to detect these appearances 

 without considerable practice, because they belong to a 

 class of impressions which we habitually train ourselves 

 to disregard. But by means of a little device which I 

 published a few years ago, the phenomenon may be 

 easily demonstrated to almost anyone. 



The beautiful effects produced by the rotation of a 

 vacuum tube when illuminated by a series of discharges 



Fig. 3. 



from an induction coil, are well known. The tube is 

 generally attached to a horizontal axis, which is turned 

 rapidly by means of a multiplying wheel ; the images 

 due to successive discharges, which, if the tube were at 

 rest would be superposed, are thus caused to occupy 

 different parts of the retina, and the result is the appear- 

 ance of a gorgeous revolving star. But if the tube is 

 caused to rotate very slowly, making about one turn in 

 two or three seconds, there occurs a different and very 

 curious phenomenon. The luminous images of the tube 

 are almost superposed, forming a bunch which is slightly 

 spread out at the ends. But about 40" behind the bunch, 

 and separated from it by an interval of darkness, comes a 

 ghost. This ghost is in shape and size an exact repro- 

 duction of the tube ; it is very clearly defined, and is of a 

 uniform bluish-grey tint. If the rotation is stopped, the 

 ghost still moves slowly on, and after the lapse of about 

 half a second disappears in coalescing with the luminous 

 tube. The phenomenon of the ghost is clearly due to a 

 succession of after-images, which are perceived a short 

 time after the retina has been impressed by the flashes 

 from the vacuum tube ; and a similar physiological 

 action, I think, explains— at least in some cases — the 

 apparent reduplication of a flash of lightning. 



Within the last year or two there has been a great deal 

 of rather lively controversy concerning the protection of 



