Although we commonly think of the human senses as very acute, yet in reality they possess many imperfections. For instance, our vision is too slow to follow the excessively rapid and brief movements of insects. Strive as one may, one cannot detect the flapping motion of a fly’s wing, or follow the different methods of flying practised by the dragon-fly and the bee. I have spoken early in this work of Marey’s wonderful researches, which were spread over a prolonged period, and carried out with the express object of extending our knowledge of animal motion.

Marey’s experiments, however, were limited in their scope, as he soon realised. The dry glass plate was not a convenient medium for recording the impressions of rapid motion, while extreme difficulties were encountered in connection with the illuminant and the exposure. These drawbacks became baffling when it was attempted to record the movements of insects. He endeavoured to solve the problem by concentrating a pencil of brilliant sunlight upon a condenser, so as to secure such a powerful luminous cone of light as to enable an exposure to be made in 1/42,000th of a second.

Marey wrestled with the task in a most determined manner, and his efforts were supported by eminent physiologists in other countries; but the obstacles were so formidable and the available resources so limited that they could not arrive at any practical result. It has been left to Monsieur Lucien Bull, the accomplished assistant director of the Marey Institute, to overcome the difficulty. Through his courtesy I am enabled to describe his fascinating researches, as well as the peculiar and efficient apparatus he has evolved for the purpose. The beautiful and highly interesting results he has achieved are shown in the accompanying illustrations.

It was obvious at the outset that the familiar cinematograph camera and its system of operation were unsuited to recording such excessively rapid movements as take place in the oscillation of a fly’s wing. Intermittent motion was quite out of the question. No device working on this principle was capable of enabling one hundred exposures or more to be made in the short space of one second. If it were attempted the film would only be torn and twisted before it had moved twelve inches. In its place continuous motion on the part of the film was imperative, and finally this requisite was supplied in a decidedly novel manner.

The general characteristics of the apparatus conceived and fashioned by Monsieur Bull may be gathered from the illustrations. As the usual illuminants were unsuited to photography of extremely rapid motion, recourse was had to the electric spark, which is of tremendous luminous intensity. These sparks are generated at uniform intervals and as rapidly as the exigencies of the experiment demand. In order to grasp the details of the installation a vertical sectional diagram, Fig. 19, is given, and by its means the design and operation of the apparatus may be gathered.

Instead of the ordinary camera there is a cylindrical wheel R, mounted rigidly upon a shaft supported on brackets at either end. The flat rim of this wheel carries the sensitised film, which, as the wheel is 13½ inches in diameter, is 42½ inches in length. This band is of the standard width and is sufficiently long to receive fifty-four pictures of the ordinary cinematographic dimensions during one revolution of the wheel. As the pictures are only three-quarters of an inch in depth, the deformation arising from the impression being made upon a curved surface is so slight as to be unworthy of consideration. The arrangement adopted has the advantage of enabling the wheel to be rotated very rapidly, so that consecutive pictures may be taken at very brief intervals of time.

As the work is carried out in the laboratory in full daylight, the wheel carrying the ribbon of sensitised film is enclosed in an octagonal light-tight box B, the upper half of which is hinged so that when the box is dismounted and taken into the dark-room it can be opened easily to permit the exposed film to be withdrawn and a new strip inserted.

The shaft upon which the cylinder revolves is fitted with an interrupter I, whereby the primary circuit of the induction coil A may be broken at regular intervals during the revolution of the wheel. If desired, as many as 2,000 interruptions can be made in one second, by rotating the wheel at a very high speed, if the necessities of the experiment demand such a high number of exposures; each interruption producing an electric spark in the spark gap E placed behind the condenser C. This condenser converges the luminous rays into the objective O, which is mounted in front of the travelling sensitised ribbon.

The lens is mounted in a small box V attached to the front vertical face of the octagonal box, the latter being pierced at this point to permit the light to pass from the objective to the film behind. In this lens box, between the back of the objective and the exposure aperture, there is a mirror M, attached at its top to a horizontal rod having a milled screw head. When the mirror is in the position shown in the diagram, the image reflected therein through the lens is thrown on to a ground-glass screen D, set in the top of the lens box. This serves consequently as a view-finder. When the exposures are to be made the screw controlling the mirror is turned, and this swings the mirror upwards like a flap until it lies flat against the under side of the ground-glass D, forming a light-tight joint.

The wheel carrying the sensitised film is driven by an electric motor, the shaft of the wheel being extended beyond the box on one side to carry a small grooved pulley, the drive from the motor being transmitted through a small belt. The motor may be driven at varying speeds, so that the number of exposures per second may be varied according to the revolutions of the box completed in that period of time.

The interrupter—also mounted outside the film-box—whereby the make and break in the primary circuit of the induction coil is controlled, and consequently the frequency of the spark between the points E in the secondary coil circuit, is a disc of ebonite, into which are compressed fifty-four strips of copper spaced equidistantly—each strip corresponding to a picture on the film—which are pressed by two metal brushes as in the commutator of the ordinary dynamo. As the copper strips in the ebonite disc pass beneath these two brushes the electric circuit of the induction coil is opened and closed, thereby producing an electric spark in the secondary circuit of the coil. The intensity of the spark is augmented by means of a small condenser L, which is placed in parallel in the secondary circuit.

The sparks are produced between two pointed magnesium electrodes, less than 1/12th of an inch in thickness, while the spark is about 1/25th of an inch in length. The spark is very rich in the ultra-violet rays, which possess a powerful photographic quality. In order that these rays shall not be absorbed during their passage through the condensers C, the latter are made of Iceland spar. To secure improved results, a small condenser c is sometimes placed immediately in front of the spark-gap E. The general arrangement of the apparatus for operation is shown on page 266, the whole being mounted upon a bench with the induction coil upon a shelf beneath.

The photographs obtained in this manner, however, are purely of a silhouette character, and often it is very difficult to interpret correctly the movement of the wings of an insect from such a result. In order to obviate this drawback, Monsieur Bull introduced a stereoscopic system, wherein two lenses are mounted side by side before the film box, with two spark-gaps in the same circuit. This enables two sparks to be produced simultaneously with each interruption of the primary circuit, to give two images of the object upon the sensitised celluloid films.

In order that the two exposures should be made upon the films exactly at the same time, a special type of shutter had to be evolved, whereby the exposure apertures were opened simultaneously at the critical moment when the cylinder commenced to revolve, and which closed in concert when the rotation was completed, because, as the films were travelling continuously, there was no necessity for an alternate closing and opening shutter movement, as is required in ordinary cinematography working upon the intermittent motion principle. The interval between the sparks acts in the same manner as a shutter swinging across the lens, and serves to secure a succession of instantaneous pictures upon the films. The images are obtained in such rapid succession that there is no possibility of the films becoming fogged through the objective apertures being open the whole time the wheel completes a revolution. The two sparks being in the same circuit, they must be produced in absolute synchrony.

The crucial question was how to open and close the shutter simultaneously at the critical moments. This was solved in an ingenious manner. The shutter itself is made of brass and is placed close to the film. There are two apertures, side by side, corresponding to the size of the image upon the cinematograph film.

The operation of the shutters is shown in Fig. 20. When the cylinder is at rest, the exposure holes are closed by a single curtain A, consisting of a thin sheet of steel of sufficient length to cover the two holes. It is held in position by means of an electromagnet controlling an extended spring. When the cylinder commences to revolve the spring connected to the shutter is released under an electric impulse discharged through the electromagnet. The shutter falls, exposing the two apertures f1 and f2. When the cylinder has completed its revolution, another electric impulse releases a second steel curtain B, held in position by a second spring, controlled by an electromagnet, so that it drops also and falls over the exposure holes. This system is both simple and positive in its operation, and it may be pointed out that this control is quite independent of the interrupter, which works in conjunction with the electric spark.

The interval of time elapsing between each picture is determined by means of a tuning fork making 50 double vibrations per second, which operates an electric signal. This tuning fork is set up in such a manner that the ends of its vibrating tongues are photographed upon the film throughout the experiment. As the vibrations of the tuning fork are known, it is only necessary, in order to determine the interval of time between each exposure, to count the number of photographs taken successively during a complete vibration of the tuning fork. As a result of experiment, however, it has been found that the ear can be trained to determine with astonishing accuracy the speed at which the apparatus revolves—and consequently the number of pictures taken per second—by the succession of sparks produced by a tuning fork the vibrations of which are known. This means that in conjunction with the photographic record, the speed at which the exposures were made can be determined without effort, and this velocity can be varied very easily. In addition, a measured scale, engraved on glass, is placed in the field of the lens, whereby the investigator is enabled to determine the exact displacement of the insect in the field of vision within a given period. Such is the apparatus, devised so far back as 1904, with which Monsieur Lucien Bull has accomplished some remarkable work of incalculable value to science, and with striking precision.

I will now proceed to explain how the photographs are taken. In the first place, in order to obtain natural and conclusive data regarding the flight of insects, it is imperative that they should be cinematographed while in full liberty, but the point arose as to how to control the instrument so that the camera and film did not commence to revolve until the moment the insect entered the field of the lens. The habits of insects vary greatly. Some fly off immediately they are released, while others hesitate for a minute fraction of a second. As the apparatus makes only one complete revolution at a time, and that in the fraction of a second, the control has to be of such a finely adjusted character that the record obtained is of movement purely and simply, and not of the insect in a quiescent state preparatory to flight.

Another question was how to induce the insect to cross the field of the lens. All insects instinctively fly towards a light. The apparatus therefore was set near a window, the insect being released from the side away from the window, so that in order to reach the light it had to traverse the field of the lens. As the latter is very small, it was essential that the release should be carried out in such a way that the insect did not fly above or below the field of the lens during exposure of the film.

At first sight these obstacles appeared insurmountable. It was obvious that the release of a fly from the hand would be too slow and uncertain, while the movement of the insect after securing its liberty would be unnatural, because no matter how delicately it might be handled, there would be the liability of injuring its fragile frame. The governing point was to devise ways and means of opening the shutter just at the moment the insect started to fly from one side of the lens field to the other, under completely natural conditions.

This delicate problem was resolved by Monsieur Bull in an ingenious manner, but not before he had carried out innumerable experiments attended by dispiriting failures. At last he succeeded in evolving means whereby the insect automatically and instantaneously opened the shutter of the camera at the moment it started to fly. The artifices by which this end was achieved are shown in Fig. 21. The system, however, had to be modified for different types of insect. That marked A was used for the ordinary house-fly and for dragon flies. It comprised a small pair of pincers, or clamp, which held the fly firmly captive by means of a small electromagnet, but in such a way as not to inflict the slightest injury. This clamp was introduced in the stereoscopic shutter electrical circuit. The two legs of the clamp have a natural tendency to fly apart under the action of a spring, but are held closed by a tooth at the end of a rocking arm, mounted on one leg, engaging with a fixed tooth on the second leg of the clamp. The latter is placed in an electrical circuit with the shutter of the camera. When all arrangements are completed, the experimenter closes this circuit. This action causes the electromagnet of the clamp to pull down the projecting end of the toothed arm; the two legs are allowed to fly apart, liberating the insect, and as the clamp releases the fly the stereoscopic shutter is opened.

This system, however, was of no avail in connection with the Hymenoptera group of insects, since wasps, bees, and similar insects hesitate slightly before they seek safety in flight. Consequently the device B was evolved. This consists of a small glass tube, in which the insect is introduced at one end. The opposite end is cut obliquely, and is fitted with a very light hinged shutter of mica, having a fragile spring, which covers about one-half of the opening. The spring closes the electric circuit operating the stereoscopic shutter of the camera.

Let us suppose a bee is to be cinematographed in flight. It is pushed head foremost into the free end of the tube, which is large enough to carry it comfortably, and the half-closed mouth is pointed towards the window. The insect naturally endeavours to approach the light, and accordingly crawls along the tube until at last it reaches the mica shutter, beneath which it endeavours to escape. As it crawls out of the tube it lifts the mica flap, and the circuit is broken. But the shutter does not open, because at this moment the operator himself closes the contact. By this time the insect has emerged a sufficient distance from the tube to complete its final preparations preliminary to flying away. Just as it springs from the tube the mica shutter falls, the electrical circuit is closed once more, the shutter is opened, and pictures of the bee on the wing are recorded upon the celluloid film.

For the Coleoptera group of insects—beetles—where there is still a more marked hesitation before flight, the device C was prepared. In this instance Monsieur Bull compels the weight of the insect, which is relatively heavy, to establish the necessary contact to operate the shutter. A glass tube without an oblique mouth is used for this purpose, and instead of the mica flap, a very light horizontal plate of aluminium, balanced by a counterweight at one end, is passed through the tube. This plate is mounted on a central pivot, and extends a certain distance from either end of the tube. The counterweight is slightly lighter than the weight of the insect, and rests in contact with a platinum point. The beetle is introduced into the tube at this end, and crawls along the rod towards the opposite end. When it has passed about half-way through the tube, and has crossed the fulcrum of the beam, it causes the contact end to rise like a see-saw, thereby breaking the electrical contact. In this case, as with the device B, the operator recloses the break in the circuit. The beetle continues its journey, and finally emerges from the tube upon the flattened end of the beam, on which it completes its arrangements to fly away. Directly it leaves the platinum beam, the latter, under the action of the counterweight, falls upon the platinum point, re-establishes the contact, the circuit is closed, the shutter is opened, and the flight is caught upon the sensitised band.

Monsieur Bull has devised a wide variety of these ingenious appliances, whereby he is able to secure the flight of any insect, and by the employment of which the chances of failure are reduced to the minimum. Indeed, when the apparatus is set up correctly, is in the hands of a skilled operator, and every arrangement has been completed satisfactorily, failure can only result from one cause—the refusal of the insect to fly away. The best results are obtained from insects which are in a fresh, healthy condition. If they have been imprisoned for too long a period, or are fatigued, the chances are that the results will be very disappointing. The records which Monsieur Bull has secured, showing insects in flight, illustrate some very interesting facts, and are of far-reaching value from the scientific point of view. At the present moment they are of additional interest owing to the absorbing fascination of aviation, inasmuch as they enable us to study with ease the movement of the wings of powerful, speedy fliers, which hitherto has been impossible under natural conditions, owing to the excessive velocity with which the wings move, and the brief character of the motion. With such an apparatus as I have described, any very rapid motion can be cinematographed, for the system is very elastic, and capable of very extensive application.

Another vast field of research which has been opened by the cinematograph is the study of the flight of projectiles. This subject has occupied earnest attention among military authorities for some time past. A few years ago Professor Vernon Boys carried out some wonderful experiments in this direction, and produced some very striking results. His work has been continued recently, by cinematographic means, by an eminent German investigator, Dr. C. Cranz, professor at the Berlin Military Academy. He has succeeded in accelerating the exposures of the film to such a degree that 500 consecutive pictures can be taken in 1/10th of a second, the exposures varying from 1/1,000,000th to 1/10,000,000th part of a second, the last-named period being such an infinitesimal fraction of time as to be beyond human comprehension. The pictures obtained are of the standard dimensions, and a striking feature, despite the tremendous speed at which they are taken, and the extremely brief exposure, is the clearness and definition obtained.

In this instance, also, the electric spark is called upon to serve as the illuminant to enable the images to be recorded upon the sensitised band, but the means whereby the interruption of the primary circuit of the induction coil is secured differs materially from that practised by Bull. Obviously the film travels with a continuous, instead of with an intermittent, motion, the sensitised band in this instance being run over two steel cylinders. The tremendous speed at which the film moves may be gathered from the fact that, at the rate of exposures made, over 280 feet of film must pass before the exposure aperture in the short space of one second—more than 2½ miles per minute.

The apparatus evolved by this German investigator is more complicated than that of his French contemporary, and the method of operation is widely dissimilar. The arrangements, too, for preventing the film receiving more than one series of pictures—the film is rotated before the exposure aperture in the form of an endless band—demanded a special electrical contact control system, while the manipulation of the apparatus varies according to the character of the experiment.

The results obtained are startling. Although taken at such a tremendous speed, the pictures when thrown upon the screen under normal conditions enable every motion to be followed quite easily because everything moves slowly. For instance, one can see the hammer of an automatic pistol fall, and follow exactly what takes place during the whole firing operation, and the ejection of the spent cartridge. Similarly, when the pistol is fired, and the bullet is photographed as it emerges from the muzzle or when it strikes, passes through and emerges from a steel plate, the movement can be followed with complete facility, as it appears to move across the screen no more rapidly than a person walks, because, although the exposures are made at the rate of 30,000 pictures per second, they are projected at a speed of only 16 pictures per second. By this means analysis of extremely rapid motion in all its details can be effected—a result of far-reaching possibilities to the study of ballistics.

Facilities are provided to enable the progress of the missile, both vertically and horizontally, to be measured, so that the speed of the projectile may be determined with unimpeachable accuracy. Fluctuations in the speed of a missile can be ascertained and investigated. Suppose the speed of the projectile is measured as it emerges from the arm, and again as it reaches the end of an extensive free trajectory; losses in velocity due to resistance of the air and other causes can be calculated. Individual pictures, when enlarged—and the sharp, well-defined character of the images on the band enables this enlargement to be carried out to a very wide limit—supply a ready means to investigate any particular phase and phenomenon at close quarters.

It is not generally known that about twelve years ago the British Government set up such an installation at Devonport to investigate the phenomena of bullets in flight. Here also illumination was carried out by means of the electric spark, but the rate of exposures was considerably slower. The results of the work of Monsieur Bull and Professor Cranz, however, has opened up another and more wonderful province for the cinematograph, and the investigation of rapid motion. The efforts of these two scientists prove conclusively that motion photography is capable of recording the most rapid movements known with perfect success, although the circumstances may demand such extraordinarily short exposure as the 10,000,000th part of a second.