PHOTOGRAPHS OF A GALLOPING HORSE.

A

BOUT two years ago I heard for the first time of a photo

graphic achievement which seemed to me at the time scarce credible, and which I was presently assured by one of our ablest English photographers was absolutely outside the bounds of possibility, to wit, the photographic presentation of a galloping horse. Of instantaneous photography, so called, I had of course heard, and I had seen the process in operation. But I knew that the actual exposure in what is called instantaneous photography is not less than a second, even in that arrangement which was called some ten or twelve years ago pistolgraphy. Again, I knew that the sun had been photographed in a period certainly not exceeding the 1,000th part of a second. But the shortness of the exposure in that case was a necessity, instead of involving a difficulty; for the brightness of the solar image is such that an exposure of the tenth or even the hundredth part of a second would suffice to entirely "burn out" the details of the photographic picture. To photograph a galloping horse, however, with distinctness, requires on the one hand an exposure of much less than a second, or even than the tenth or hundredth part of a second; while, on the other hand, the luminosity of the image cannot, under any circumstances, be greater than that which, when ordinary photographs are taken, involves an exposure of several seconds at least.

As to the first point, it is easy to see that an exposure of a second would result in entirely blurring the outlines of the horse's limbs. A galloping horse advances ordinarily at the rate of a mile in less than two minutes. In the photographs of which I had heard, the rate mentioned was a mile in 1m. 40s., or thirty-six miles per hour. Taking the last-named rate, or a mile in 100 seconds, the galloping horse advances one hundredth part of a mile, or nearly eighteen yards, in a second, and therefore, as a horse at rest occupies a width of less than three yards, it is hardly necessary to say the picture obtained from an exposure of one second would be a mere confused

blur. The image obtained in the tenth of a second would be no better, as the blurring would correspond to a width of nearly two yards. In the hundredth part of a second the image would be blurred to a width corresponding to more than half a foot-so that, although the picture of the horse as a whole might be perhaps just recognisable as a horse, the limbs would be confused beyond recognition. To get a picture which should show the limbs of a galloping horse with anything like distinctness, the blurring should not exceed a width corresponding to one inch in the life-size image of a horse. Now, in what precedes I have only taken into account the forward motion of the horse as a whole; but in considering the definition of the limbs, we have to remember that these are not only advancing with the body, but are moved also in relation to the body, and that when the limbs are being thrown forward, this forward motion is added to the advancing motion of the body. Now, the forward motion of the limbs varies in rate, from nothing when the limbs are farthest forward and farthest back, to a maximum somewhere near the middle of their forward sweep. This maximum cannot be less than the advancing motion of the horse, and is probably much greater. As we must add this forward motion to the advancing motion of the horse as a whole, we get for the maximum forward motion of a limb (meaning now the full forward motion, not only the motion relatively to the body) twice the advancing motion of the horse. We have seen that with an exposure of one second the blurring of the body of the horse would have a width corresponding to half a foot in the life-size image of a horse. The blurring of the limbs would vary from nothing to a width corresponding to a foot. That the blurring, then, should nowhere exceed a width corresponding to an inch, the exposure should not exceed the 1,200th part of a second in duration. As a matter of fact, satisfactory pictures were not obtained until the exposure had been reduced to the 2,000th part of a second, and in later pictures the exposure has been reduced to the 5,000th part of a second.

And here, in passing, I may answer an objection which will occur perhaps to many readers. I remember that after mentioning in a lecture at Sydney, New South Wales, the brief exposure of Janssen's

In the case of a carriage, we get in the motion of the wheels what corresponds to the relative motion of the horse's limbs. In this case, we know that the relative forward motion of the top of the wheel, and the relative backward motion of the bottom of the wheel, are each equal to the advancing motion of the carriage, so that the top of the wheel is advancing twice as fast as the carriage, while the bottom of the wheel is momentarily at rest.

solar negatives, I was asked by one of the chief photographers of New South Wales, who had been present, how I could venture to speak of an exposure of the 1,600th part of a second, when no means could possibly be devised for measuring so short a period of time. I was able to reply that not only had Janssen been able in the most satisfactory manner to measure the exposure of his plates to the solar image, but that science had been able to measure periods of time so short as the 100,000th, and even the 200,000th part of a second. Nay, Wheatstone claims, and not without good reason, that, when attempting to determine the duration of a lightning flash, he measured periods very much shorter even than this. It sounds at first hearing altogether incredible, and indeed absurd, that men should pretend to measure by optical and mechanical means (for so has the task been achieved) a period which is a very small fraction of the duration of a luminous impression on the eye. Yet in reality this has been done by taking advantage of the very circumstance which seems at first sight to render it impossible. The method is so ingenious, and at the same time so simple, that it will be well to consider it here as an introduction to the less minute subdivisions of time involved in the processes which form the subject of this essay.

Conceive a rather large disc of ebony, round the edge of which are inlaid radiating lines of silver wire, exceedingly fine. Say, for instance, that there are 1,600 equidistant radiating lines, or in each quadrant 400, so that each centigrade degree (100 to the quadrant) is divided into four parts. If each wire is the hundredth of an inch in thickness, and the disc is one foot in diameter, the black space between the ends of the wires will be one-hundredth and a quarter (of a hundredth) in width. Now, suppose this disc set in rapid rotation, making, for instance, a hundred rotations per second. Then, in the 160,000th part of a second, one of the radiating wires will be carried to the position which, at the beginning of that short period, had been occupied by its next neighbour. But the forward edge of a wire will be carried to the position which had been occupied by the backward (or following) edge in a shorter time still-manifestly in five-ninths of the short period; for the breadth of the black space between the wires is five-ninths of the distance from centre to centre of successive wires. Thus, if the disc is whirling in darkness, and is suddenly lit up by a flash of lightning, and the flash lasts five-ninths of the 160,000th part of a second, or lasts one 288,000th of a second, the disc will appear as if bordered by a continuous ring of silver; for

during that time every part of the edge will have been occupied by lightning-lit silver, and as the eye retains a luminous impression for fully one-tenth of a second, the light from every part of the edge of the disc will appear to form a single image, in which the spokes of wire will not be separately discernible. If the lightning flash lasted half that time, the black spaces would be discernible, but would seem to be but of half their real width, half their width being cut off during the continuance of the flash. If the flash lasted a fourth of the above-mentioned time, only one-fourth of the width of the black space would be cut off, so that its width would appear but threefourths of what it really was, and so forth for yet shorter periods. But this will suffice to show that Wheatstone could measure by this method, as he claimed, the millionth part of a second. For manifestly the eye could readily detect the diminution of the black spaces by a full fourth of its amount, and this reduction (on our assumptions as to the size of the disc and the rate of its rotation) would be produced if a lightning flash lasted but one 1,152,000th, or less than the millionth part of a second. Thus, when Wheatstone stated, as the result of his experiments, that a lightning flash does not last the millionth part of a second, he was not (as some rashly asserted) announcing over-confidently what could not by any possibility have been established by evidence, but was, in fact, simply asserting what he had satisfactorily proved. Yet, how wonderful it seems at first that science should be able to say, as it did in this case, that a luminous appearance, visible for fully the tenth of a second, lasts in reality less than the 20,000th, or even than the 100,000th, part of that time.'

We see, then, that it is not only possible, but an easy matter, to measure periods of time much shorter than the 1,000th or 10,000th part of a second. But it might still seem marvellous, and in fact it is, that science should be able so to arrange matters that in such a minute period of time an image should be taken which shall be clear and well defined in all its details. Yet this has been achieved,

1 Within a few hours of writing the above lines, I witnessed at the observatory of Dr. Henry Draper, of New York, a very simple experiment illustrating the instantaneous character of the electric spark, and also the intermittence of a luminosity which, as judged by the eye, appears persistent. While the electric discharge was taking place in a series of rapidly following sparks, the hand held steadily in front of the light appeared to be quite steadily illuminated; but if the hand was rapidly fluttered about, a multitude of distinct images of the hand were seen, producing an appearance as of a multiform hand with multitudinous (and ever varying) fingers attached to it,-the explanation being, that the hand was Successively visible and invisible, and many successive images were seen in different positions during each tenth of a second of the duration of luminous impressions.

and some of the results of the application of this process have now to be considered.

In the best paintings of horse-races, charges, the hunting-field, and so forth, we have what may be regarded as a conventional view of the horse at full gallop. He is shown with the two fore legs thrown well forward and the two hind legs thrown well back-in the attitude, in fact, which is indicated by the French expression ventre à terre, applied to an animal at full gallop. Anyone who has watched a race or a charge of galloping horses will certainly be prepared to affirm that this attitude is one of those which a horse assumes in galloping. It is, of course, to some degree absurd that this one attitude, which is only (even on this assumption) assumed at certain definite instants by the horse at full gallop, should be presented as the only or almost the only attitude recognisable in a group of galloping horses. Still, the idea generally entertained by those who study pictures of the kind is that this attitude is the most characteristic, and the one best suited for delineation. Accordingly, paintings and drawings of galloping horses which present this attitude and no other, are amongst those most admired by the artistic world.

So soon, however, as we test by instantaneous photography the movements of a horse, we find that this admired and presumedly characteristic attitude is not one which really characterises the gallop. Not only is this the case, but the attitude is actually never assumed at all by a horse either in this or in any other gait. And, on the other hand, we find that positions are assumed by the galloping horse which no one would for a moment have supposed possible.

The positions shown in Mr. Muybridge's photographs are eleven, and these include all the movements made in one complete stride. It requires some care to distinguish the movements of the different legs. Let us follow the movements seriatim.

The first position of the series is that shown in Fig. 1. Here the

may be followed. It must be remembered that this picture does not