PHOTOGRAPHING LIGHTNING WITH MOVING CAMERA. 123 



branches of the dark flash would have been obliterated by the other 

 rushes following. The ert'ect of halation and solarization was also 

 considered, but rejected. There was thus but one way to account for 

 the phenomenon, namely, that the flash must have given out light 

 of a wave length much shorter than the wave lengths of visible light 

 and with a power sufficient to render the portion of the plate struck 

 by it nonsensitive to ordinary light. Such a flash would appear 

 black on a partially illuminated background or be invisible. 



Dark flashes have been observed by the writer on several occasions 

 i.nd only when raining very hard. They appear to the eye the same 

 as the accidental image produced after looking at a bright flash. 

 Such an image may be retained in the eye for quite a while after, but 

 can not easily be confounded with a real flash. 



Two other pictures of dark discharges have been obtained with 

 the moving camera, but are not so prominent; tliey likewise show the 

 hrst rush as black, but without a bright core and with no side 

 branches; one is of a horizontal and the other a vertical flash. 



Below is a table of the measurements of this flash. The angle of 

 the lens was G0°. Time of rotation of camera, 1 revolution in 10 

 !-econds. Width of plate, 127 millimeters. The width of the whole 

 flash, 4S millimeters. Time for the whole flash, 0.()24 second. The 

 iiverage distance between the rushes, 1.2 millimeters. Average time 

 for each rush, 0.015G second. The plate used was a " Standard '' 

 ordinary plate. The contrast of the picture has been increased by 

 means of a double-contact print with lantern-slide plates. The 

 developer was '' Rodinal.'' Time for developing, five minutes. 



Liyhtning flush {flu. 5). 



Designation. ■ 



From fifteenth to si.xtcenth 

 rush 



From sixteenth toseventoenth 

 rush 



From seventeenth to eight- 

 eenth rush 



From eighteenth to nineteenth 

 rush 



Width of nineteenth rush 



From nineteenth to twentieth 

 rush 



From twentieth to twenty-first 

 rush 



From twenty-first to twenty- 

 second rush 



From twenty-second to twenty- 

 third rush 



Width of tweiilv-third rush.... 



Dis- 

 tance. 



Mm. 



0.5 



.6 



.5 



1.3 

 .6 



2.0 

 .1 



Time. 



Second. 



0. 0065 



.0078 



. 0065 



.0169 

 . 0078 



. 0182 



.0091 



.0182 



. 0260 

 .0052 



