178 



♦ KNOWLEDGE ♦ 



[July 1, 1889. 



might be expected. Photograph No. 6, Plate I., is made 

 from a trans])aroncy kindly lent me by Dr. Hoffert ; it 

 will be seen that similar flashes are shown upon the plate 

 at a considerable distance apart, joined by nebulous fila- 

 ments. Dr. Hofiert says, in a letter to Mr. Marriott: — 

 " With regard to the questions in your letter, I. can give 

 you the following general account of the way in which the 

 photograph was taken. I was standing on a balcony, the 

 rain having ceased. The storm had approached close and 

 was all around us. At tlie point to which the camera was 

 ilirected were occurring numerous brilliant discharges, at a 

 distance of, I should think, half a mile to the east-north- 

 east of my house, which is close to Ealing Bi'oadway Rail- 

 way Station and on the Green. The flashes seemed to my 

 eye single. The camera was moved to and fro so as to 

 make a complete period in about three-quarters of a second. 

 I have thus estimated the period between the discliarges as 

 somewhere between one-tifth and one-tenth of a second. The 

 period that the plate was exposed was between a quarter 



f 

 and half a minute. The aperture was -^ , and a rapid recti- 

 linear lens was used." 



Dr. Hoffert has kindly lent me two transparencies from 

 this photograph which, when placed one over the other and 

 slipped from side to side, show that the three great flashes 

 in the lower part of the jiicture are exactly of the same 

 shape. They each have a double brighter head which, 

 when magnified, shows a series of knots or beads. The 

 parts of the flashes which turn suddenly away from or 

 towards the observer leave the brightest traces joining one 

 flash with the other. The question arises, are these traces 

 due to a glowing of the incandescent air in the path of the 

 discharge, or to the light of a series of smaller discharges 

 which continue to flnd their way down the path of discharge 

 in the interval between the great flashes. If the glow is 

 due to incandescent air we should expect to find itgi-adually 

 and evenly subsiding as the air or gases cool, and this is 

 certainly not the case. 



On the right-hand side of the lower part of the middle 

 flash a series of increases and decreases in the brightness 

 of the nebulous field due to the glow can be traced, showing 

 that the glow varies in brightness between the flashes. The 

 glow, whatever it is due to, lights up again and cools several 

 times in the interval between the great flashe.s. 



This has produced a very curious eflect in the case of the 

 upper flash, of which there are three images at the top of 

 the photograph. The left-hand image is seen to have a 

 black line down it. In fiict it has been described as a dark 

 flash. When examined closely with a lens it is seen to be 

 bordered by a series of tufts parallel to the traces joining the 

 flashes. Tiiese tufts correspond exactly in position with 

 brighter beads or knots on the other two bright images of the 

 upper flash. And the dark channel is seen to be a division 

 between two somewhat brighter fields produced by a glow 

 which followed in the minutest details the curves as well as 

 the knots and beading of the brighter flashes. The channel, 

 in fact, corresponds to a period when the series of smaller 

 discharges down the path of the flash ceased. I do not 

 assert that all dark flashes are of this character, but here 

 we evidently have one means by which a dark channel 

 curving in tlie characteristic manner of lightning may be 

 produced. 



It is worthy of remark that the dark flash shown 

 in photograph No. 4 in the June number was taken on a 

 plate which evidently moved during the exposure, as is 

 ])roved by the superposed images of the roofs at the bottom 

 of the picture. Mr. E. S. Shepherd, by whom the photo- 

 graph was taken, informs me that the camera was held by 

 the hand on a window-ledge, and it is quite clear that the 



camera moved between the flashes which illuminated the 

 roofs, though Mr. Shepherd was unconscious of any move- 

 ment. A similar remark also applies to photografih No. 2 

 in the June number, also taken by Mr. Shepherd. It is one 

 of the ribbon flashes along which runs a black flash or 

 channel following the sinuosities of a parallel bright flash 

 at the upper edge of the ribbon. In this case the camera 

 seems to have been moved in a vertical direction. Jlr. 

 Shepherd informs me that the camera was held by the hand 

 on the edge of the window-sill, and directed upwards, and 

 he is unconscious of having flinched when the flash came ; 

 but nearly two years have now elapsed, and his attention 

 was not then specially directed to the matter. 



We rejected this theory of the origin of libbon-flash 

 photographs in the article in the J une number, because we 

 then believed lightning flashes to be as instantaneous as the 

 electric discharges which can be experimented upon in the 

 laboratory, and because it seemed that the strire- across tlie 

 ribbon flash in photograph No. 1 in the June number were 

 not quite parallel on diflerent parts of the jilate ; but I have 

 since had an opportunity of examining the negative of this 

 flash, which was taken on a large plate, and I have satisfied 

 myself that the want of parallelism is due to the optical 

 distortion of the lens with which the photograph was taken. 



As far as I have yet learnt, the photographs of broad 

 ribbon flashes have all been taken with instruments held in 

 the hand, or held against a fixed support, but not screwed 

 to it. If we account for all the photographs of ribbon 

 flashes by an assumed motion of the plate during the ex- 

 posure, we are still confronted by the observations of those 

 naked-eye observers who believe that they have seen ribbon 

 flashes in the heavens. The observation must be a very 

 difiicult one to feel any certainty about. It is only since 

 I'ibbon flashes have been photographed that a few observers 

 think that they have seen ribbon flashes in the heavens, 

 and we have to set against them a much greater number of 

 observers who have watched for ribbon flashes during the 

 same storms and seen none. It is very easy to deceive our- 

 selves with respect to any observation which lies near to 

 the limits of our senses — and there is another possible 

 exjjlanation. If the most sensitive part of the eye were 

 not turned in the direction of the first flash, the eye would 

 involuntarily turn so as to place the most sensitive part of 

 the retina in the right direction ; and if the second flash 

 occurred before the image of the first flash had faded from 

 the retina, it is very conceivable that an observer would 

 suppose himself to see two parallel flashes. It is known 

 that the sensitiveness of the retina of diflerent persons for 

 such after-images diflers greatly, and hence such a pheno- 

 menon, though noticed by one, might not be seen by another. 

 Similarly, the appearance of the dark flash would be 

 accounted for by a coloured persistent image of a previous 

 lightning flash remaining on a sensitive retina. W^e are all 

 familiar with such after-images after staring at the sun ; a 

 sensitive eye may see them after looking at a lightning 

 flash. 



In addition to the broad ribbons referred to above, a 

 narrow ribbonlike apjjearance may be ciiused at the edge of 

 the plate by lenses which give very sharp images of ordinary 

 objects ; for lightning flashes are so bright that optical 

 imperfections show themselves which would not affect 

 the .sensitive film around the image of an object of 

 ortlinary brightness. With the short focussed cameras in 

 use, the pencils of light fall very obliquely at the edge 

 of the plate, and the oblique sections of such pencils 

 just beyond the focus frequently consist of a bright point 

 with a nebulous tail, the shape of which varies greatly with 

 the optical system used. Photograph No. 10 is an illustra- 

 tion of such optical distortion at the edge of the plate. 



