July 1, 1889.] 



KNOW^LEDGE ♦ 



179 



Colonel Tupman has also sent me a negative of a lightning 

 flash, showing a very distinct ribbon form at the eJge of the 

 plate, evidently due to considerable optical distortion. He 

 has also sent me a beautifully sharp photograph of a land- 

 scape taken with the same lens set at the same focus. The 

 examination of photographs of lightning llashes taken with 

 instruments which have been well focussed affords an 

 excellent means of studying the optical imperfections of the 

 lenses employed. 



But though the lightning flashes are bright enough to 

 bring out the optical defects of the instruments, they are 

 not bright enough (certainly as a general rule) to give rise to 

 photographic reversal, such as would cause the line of the 

 bright flash to appear dark upon the plate. I come to this 

 conclusion for the following reasons. The photographic 

 images of sparks taken in the laboratory do not show photo- 

 graphic reveisal. The giant flashes in Nature's laboratory, of 

 course, might do so, but none of t he photographs of dark flashes 

 I have seen show bright borders, such as one would expect 

 to find, corresponding to the nebulous borders due to the 

 optical imperfections of the instrument. Take, for ex- 

 ample, photograph No. 8. If the light of the great branch- 

 ing flash were intense enough to cause photographic reversal 

 at its centre, the hazy border at its edges would remain 

 white. I have seen a pajter print of one such dark flash 

 with bright branches, which may possibly be due to photo- 

 graphic reversal ; but one ought to have an opportunity of 

 examining the original negative before coming to a definite 

 conclusion. The mystery of the black fl;»sh cannot yet be 

 regarded as completely solved. The further one proceeds in 

 such an inquiry the more curious phenomena crop up, 

 which need investigation. The bright beaded heads or 

 knots in the air with which many flashes appear to com- 

 mence are well worthy of study. Examples of such bright 

 heads to flashes are shown in photographs Nos. 4 and 7, as 

 well as in Dr. Hofiert's photograph. 



Especial attention ought, during a future storm, to be 

 given to measuring the time occupied by the phenomenon 

 of a recurrent flash with a revolving cylinder or band of 

 sensitive paper driven by clockwork, at a known rate, 

 within the camera. In Dr. Hoffert's interesting photograph 

 the groat flashes appear to have the form of a narrow 

 ribbon. One edge of the rilibon is ([uite sharp, and in the 

 image of the great flash in the centre of the plate theie 

 is on Dr. Hoftert's photographic transparency (but not on 

 the photographic copy published with this) a narrow line 

 between the sharp edge and the remainder of the ribbon, as 

 if the flash had consisted of an almost in.stantaneous dis- 

 charge, and after a minute interval of time another dis- 

 charge of considerable brightness, which lasted for a very 

 short period, but still sufliciently long to show evidence of 

 the motion of the plate. The dark interval between the 

 flash and the ribbon shows that there was an interval of 

 darkness, and that the ribltoii cannot be due to incandescent 

 air left in the wake of the flash. 



A recurrent flash seems to bo a very complicated sue 

 cession of discharges lasting for an appreciable part of a 

 second. The giant discharges which take place during a 

 storm between irregularly-shaped and badly conducting 

 masses probably dificr materially in character from the 

 flashes piodiued in a laboratory between good conductors. 

 In the laboratory the whole flow takes place at once. In 

 nature there seems to bo a flow or rush succeeded by a 

 dribble, which ceases or nearly ceases, and commences again 

 and again, flow after flow rushing down the same path 

 until the potential along the lino of discharge is equal- 

 ised. There seems to bo no evidence of a back rusli or 

 alternate backward and forward flash, as has bern suggested. 

 On Dr. Hoffert's photograph, and iu our plates Nos. 1 and 



2, and in the many ribbon flashes, all the parallel threads or 

 flashes thin out in the same direction. If there were 

 alternate flashes we should expect to find one thick at one 

 end, and the next thick at the other. In Dr. Hoffert's 

 photograph the left-hand flash of the lower series (which is 

 the only branched flash on the plate) .seems to have been 

 the first in point of time, and the right-hand flash of the 

 upper series of flashes seems to end with the black channel as 

 an expiring effort. 



The extreme rapidity of lightning has long been used as 

 a synonym for that which is instantaneous. In our pro- 

 verbial philosophy, " Quick as lightning " and " Like a 

 flash of lightning " have been used to denote the explosive 

 suddenness of changes which occupy less time than the 

 twinkling of an eye. But we need measures of these things 

 for a more exact philosophy. 



XOTE BY MR. W. MARRIOTT. 



I fully concur with Mr. Ranyard as to the photograjihs 

 of ribbon flashes being due to a movement of the plate during 

 its exposure to one of the recurrent flashes described in the 

 second paragraph. As far as I am aware, no photograph 

 of a riljbon flash has been obtained with a camera on a fixed 

 support. The motion of the camera in photographs 1 and 2 

 is clearly proved by the superposed images of the signal-box, 

 which seem to correspond in position and intensity with the 

 different images of the lightning flash. An appearance of a 

 narrow ribbon may also be produced by optical causes near 

 the edge of the plate, where the pencil of light from the lens 

 falls obliquely and the sensitive film is either beyond or 

 within the focus ; the section of the pencil of light is then 

 not circular, but usually consists of a bright point with a 

 nebulous tail, causing a hazy edge to the bright image of 

 the flash. In these latter cases the ribbon character is not 

 continued all across the plate, but the breadth of the flash 

 and its hazy edging vary with the distance from the centre 

 of the plate. 



I also concur as to the multiple character of many of the 

 flashes during the storm on June 6, and it seemed to me 

 that the first flash was the brightest ; this was succeeded by 

 less brilliant flashes along the same course. In one instance 

 I saw two flashes which appeared to me to be parallel, and 

 separated by a dark interval. In this aise the second flash 

 was certainly the fainter. I am not sure that the apparent 

 duplicity of the flash was not due to a motion of the eye, 

 which would involuntarily be directed so that the second 

 flash would fall on the most sensitive part of the retina, if 

 in the first instance it had not done so. 



TIGER-BEETLES.- 



By E. a. BiTi.Ei!. 



II. 



KiKlv-BEETLES not only run with great 

 agility, but readily take flight as well, and 

 hence are rather difficult to capture ; and as 

 their eyes are prominent and can take in a 

 wide horizon at one view they need to be 

 \ery warily " stalked.'' The organs of 

 flight are ;<. pair of dark-coloured membran- 

 ous wings (tig. 4), which, under ordinary circumstances, are 

 ])acked away out of sight, bcnejitli their green, horny covere. 

 Being larger than their covers, they have to bo folded up 

 before they can be packed away : this is done twice trans- 

 versely, the tip being bent round towards the ba.se, and a 

 second bend in the same direction then made higher up the 

 wing, and then the two wings, as they lie along the back, 

 partially overlap one another. Keeping these airangements 



