166 ELECTRIC-SPARK PHOTOGRAPHS. 



tin's way brought into view, we get what may in ordinary language be 

 called an instantaneous impression aud the object apj^ears clear, sharp, 

 and at rest. In the same way if we wish, with the object of obtaining 

 a permanent record, to photograph a moving body Tve must either allow 

 the eye of the camera to see through a hole for a moment, L e., we must 

 use a rapid shutter — and many such are mcU known — or we must, keep- 

 ing the photographic plate exposed and the object in the dark, make a 

 flash of liglit at the right time. As before, if the shutter is open or the 

 flash lasts so short a time that the object can not move appreciably in 

 the time, then, if any impression is left at all, it will be sharp, clear, and 

 the same as if the body were at rest. The first method, that of the 

 shutter, I do not intend to speak about to-night, but as, owing to the 

 kindness of Mr. F. J. Smith, I have with me the most beautiful example 

 that I have seen of what can be done by this method, I thought per- 

 haps I should do well to show it. Mr. Smith was in an express train 

 near Taunton, travelling at forty miles an hour, and when another 

 express was coming up in the o})posite direction at sixty miles an hour, 

 i. f., approaching him at one hundred miles an hour, he aimed his 

 camera at it and let a shutter of his own construction open and shut so 

 quickly that the approaching train was photogra]>hed sharply. There 

 is a special interest al)out this photograph; it shows one of the now 

 extinct broad-gauge engines on the road. However, this is an example 

 of the method which we shall not consider this evening.* For our pur- 

 ])ose we require what is called instantaneous illumination — a flash of 

 light. It is of course obvious that it depends entirely upon the speed 

 of the object and the sharpness required, whether any particular flash 

 is instantaneous enough. No flash is absolutely instantaneous, though 

 some may last a very short time. 



For instance, a flash of burning magnesium powder lasts so short a 

 time that it may be used for the purpose of portraiture, and while it 

 lasts even the eye itself has no time to change. PI. ii, fig. 1, represents 

 a photograph of the eye of Mr, Colebrook after he had been some minutes 

 in a dark room, taken by the magnesium flash ; the same eye taken in day- 

 light appears in PI. ii, fig, 2. The pu])il is seen fully dilated and the 

 eyelid has not had time to come down, and so we might reasonably say 

 that the flash was instantaneous; it was for the purpose practically 

 instantaneous. Yet when 1 make this large clock fiice, 4 feet across, 

 revolve at so moderate a speed that the periphery is only travelling at 

 40 miles an hour and illuminate it by a magnesium flash you see no 

 figures or marks at all, only a blur. Thus the magnesium tlash, which 

 for one purpose is practically instantaneous, is, tested in this simple 

 M ay, found to last a long time. Let me now, following Lord Rayleigh, 

 contrast the effect of the magnesium flash with that of a powerful electric 

 spark. At each spark the clock face appears brilliantly illuminated 



* I have he.ard that a canuon ball has been photographed by means of a rapid 

 shutter, but I have uo direct information on the subject. 



