168 
PHOTOGRAPHY AND METEOROLOGY. 
Fix the camera rigidly (do not hold it in the hand) and expose 
it to a part of the sky where flashes are frequent. 
As soon as one flash has crossed the field of view change the 
plate. 
Whenever possible, count the number of seconds between seeing 
the flash and hearing the beginning of the thunder. Note this 
time on the print or form. 
If you have two cameras some useful results may he attained 
by using one as described above and holding the other in the hand, 
pointing in about the same direction, but kept in constant oscilla¬ 
tion. It is hoped that two photographs of the same flash may be 
thus secured. 
Another desirable experiment is to fix both cameras in the same 
direction, change the plates in one after each flash, but leave the 
plate exposed in the second until six or eight flashes have crossed 
the field of view. 
If the camera is placed in a window this must be open, as the 
interposition of a window pane may give rise to multiple images. 
Ee particularly careful to note the exact time and direction of 
each flash photographed. f 
A rapid lens, with a stop ^ or thereabouts, should be used for 
lightning. 
