March 2. 1893] 



NATURE 



415 



ON ELECTRIC SFARK PHOTOGRAPHS; OR, 

 PHOTOGRAPHY OF FLYING BULLETS, &^c., 

 BY THE LIGHT OF THE ELECTRIC SPARK> 

 I. 



WHEN I was honoured by the invitation to deliver this 

 lecture I felt some doubt as to my ability to find a 

 subject which should be suitable, for there is a. prevailing 

 idea that in addressing the operative classes, it is neces- 

 sary to speak only of some practical subject which bears 

 immediately upon the most important industry of the 

 place in which the lecture is being delivered ; but it seems 

 to me that this is a polite suggestion that the audience 

 arc unable to be interested by any subject except that 

 particular one which occupies them daily. Now though 

 I am a comparative stranger in Scotland I have heard 

 quite enough, and I know quite enough, of the superiority 

 of the education of you, who have the good fortune to live ,_ 

 in this the most beautiful half of Great Britain, to be aware ^ 

 that, as is the case with all highly-educated men, you are ' 

 able to take a keen and genuine interest in many subjects, 

 and that I had better choose one to which I have specially 

 devoted myself, if I do not wish to expose myself to the 

 risk of being corrected. I will ask you therefore in im- 

 agination to leave your daily occupation and come with 

 me into the physical laboratory, where, by the exercise of 

 the art of the experimentalist, problems which might seem 

 to be impossible are continually being solved. I wish as an i 

 experimentalist to present to you an example of experi- | 

 mental enquiry. 



Let us suppose that for some reason we wish to examine 

 carefully and accurately some moving object travelling, if \ 

 you will, at so great a speed that, observed in the ordinary | 

 way, it appears as a mere blur, or perhaps at a speed so 

 tremendous that it cannot be seen at all. In such a case, 

 in order to get a clear view of the moving body we may 

 either look through an aperture which is only opened for 

 a moment as the body passes by, or we may suddenly i 

 illuminate the object by a flash of light when it is in a 

 position in which it may be seen. If in either of these 

 cases the hole is open, or the illumination lasts so short a 

 time that the object has no time to move appreciably 

 while it is in this way brought into view, we get what may 

 in ordinary language be called an instantaneous impres- 

 sion and the object appears clear, sharp, and at rest. In 

 the same way if we wish, with the object of obtain- 

 ing a permanent record, to photograph a moving 

 body we must either allow the eye of the camera 

 to see through a hole for a moment, i.e. we must 

 use a rapid shutter, and many such are well known, or 

 we must, keeping the photographic plate exposed and the 

 object in the dark, make a flash of light at the right 

 time. As before, if the shutter is open or the flash lasts 

 so short a time that the object cannot move appreciably 

 in'jthe 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 ex- 

 ample that I have seen of what can be done by this method, 

 I thought perhaps 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 opposite direction at sixty miles an hour, i.e. 

 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 

 photographed sharply. There is a special interest about 

 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. -' 



1 Lecture delivered at the Edinburgh meeting of the British Association by 

 C. V. Boys, F.R.S. 



- I have heard that a cannon-ball has been photographed by means of a 

 rapid-shutter, but I have no direct information on the subject. 



NO. 1218. vol. 47] 



For our purpose 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, v.hether any particular flash 

 is instantaneous enough. No flash is absolutely instan- 

 taneous, 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. The lower part of the 

 second slide (Fig. a) is a photograph of the eye of Mr. 



Colebrook after he had been some minutes in a 

 dark room, taken by the magnesium flash ; the upper 

 part is the same eye taken in daylight. The pupil 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 I make this large clock-face 

 four feet across revolve at so moderate a speed that 

 the periphery is only travelling at forty miles an hour and 

 illuminate it by a magnesium flash you see no figures or 

 marks at all, only a'blur. Thus the magnesium flash, 

 which for one purpose is practically instantaneous, is, 

 tested in this simple way, 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 illu- 

 minated and absolutely at rest and clear, and if it were 

 not that I could at once illuminate it by ordinary light 

 it would be difficult to believe that it was still in motion. 

 The electric spark has been often used to produce a 

 flash by means of which phenomena have been observed 

 which we ordinarily cannot see. For instance, Mr. 

 Worthington has in this way se^n and drawn the exact 

 form of the splash produced by a falling drop of liquid. 



Mr. Chichester Bell, Lord Rayleigh, Mr. F. J. Smith, 

 and others have used the illumination produced by an 

 electric spark to photograph phenomena which they were 

 investigating. I am able to show one of Lord Rayleigh's, a 

 breaking soap-bubble, in which the retreating edge, travel- 

 ling something like thirty miles an hour, is seen with all the 

 accuracy and sharpness that is possible with a stationary 

 object. Mr. F. J. Smith has extended the use of sparks 

 for the purpose of physiological enquiry, taking a row of 

 photographs on a moving plate at intervals that can 

 be arranged to suit the subject, and is thus putting in the 

 hands of the much-abused experimental physiologist a 

 very powerful weapon of research. I had hoped to show 

 one of these series of an untechnical character, to wit, a 

 series taken of a cat held by its four legs in an inverted 

 position and allowed to drop. The cat, as every one is 

 aware, seems to do that which is known to be dynamically 

 impossible, namely, on being dropped upside down to turn 

 round after being let go and to come down the right way 

 up. The process can be followed by means of one of Mr. 

 Smith's multiple spark photographs. However, his cats 

 do not seem to like the experiments, and he has in con- 

 I sequence had so much trouble with them that his results. 



