108 Dr. H. H. Hoffert on 



either of those on the print, the coincidence is seen to be 

 exact even to the smallest irregularities. Nevertheless, of all 

 the branch-flashes which spread out from I. only a small trace 

 exists at n in II., and none whatever in III. Sweeping across 

 the photograph and connecting corresponding points in the 

 successive flashes are streaks of light, showing that a very 

 considerable residual illumination remains between the dis- 

 charges. These streaks are especially well marked between the 

 components of the double flash VII., VIII., and are also very 

 bright along the path of the head of flash I. They are not 

 always present; for in another photograph obtained upon a 

 moving plate by Mr. Snelus, in which a flash is reduplicated, 

 there is no trace of them, the flashes being quite sharp and 

 distinct. The streaks commence abruptly with one discharge 

 and end abruptly with another. Their extension to the left 

 of I. was probably due to the camera not having quite reached 

 the end of its swing. Where they are brightest there is in 

 all cases a swelling at the part of the flash where they start. . 



At the upper left-hand corner there is a curious dark flash 

 with bright edges and short luminous streaks. The form of 

 this dark flash is exactly reproduced in the two bright flashes 

 V. and VI. Dark flashes are frequently met with in photo- 

 graphs of lightning, and have been usually ascribed to 

 reversal of the image by overexposure; but I do not think 

 this explanation applies in the present case, both on account 

 of the appearance w r hich the dark flash presents, and also 

 because as far as I can recollect the brilliant downward 

 flash I. appeared to the eye much the brightest of those that 

 occurred while the plate was exposed. There is a similar but 

 less distinct dark gap to the left of VI. ; and to the right of 

 II. are three or four faint bands parallel to it and following 

 its sinuosities. These all seem to be due to variations in 

 brightness in the luminous streaks, which are thus shown to 

 be electrical and not phosphorescent in character. 



When cnrefully compared, the forms and positions of the 

 flashes I. to VIII. are found to present such agreement as to 

 show that they must have formed a system of discharges 

 closely connected together. If V. be placed over IV. so as 

 to coincide with its left-hand border, the portion d exactly 

 coincides in form and position with the portion a of I., and 

 both a and d terminate in a curious bifurcated enlargement 

 which, when examined with a lens, shows a beaded appearance, 

 roughly sketched in fig. 3. This coincidence of form and 

 position would be difficult to account for if I. and IV. were 

 independent flashes not occurring at the same time. It is 



