FIG. 23.—ELEVEN SUCCESSIVE ATTITUDES OF A FLYING GULL.
In this series of images, traced from the originals, the distances representing the positions
of the bird in space are exaggerated to avoid confusion.
The objective turned toward the right has a slit in the center for the passage of the diaphragm which, in revolving, allows the light to pass intermittingly. When the small diaphragm makes one revolution the large one makes five revolutions, and it is then only that the apertures meet and the light passes. The bellows behind the objective allows the light to reach the sensitized film. The box is, of course, tightly closed. The focusing is done by means of a small telescope or spy glass. It is necessary at each new experiment to use a new band of film, and the substitution of rolls of films is effected in the light by means of bobbins upon which the film is rolled.
At the extremity of each band of film are glued paper bands of the same width. One of these prolongations is red and the other is black. Each of them is about twenty inches in length. Having the two colors makes it almost impossible to reëxpose a film, as one is not liable to confound a bobbin which has been used with one that has not, the color of the roll being different. Special devices are employed in the camera to render the film immovable for an instant while it receives the impression from the object. Arrangements are also provided for obtaining a uniform velocity. The use of the apparatus which we have just described permitted of seeing with what a variety of means of locomotion the various kinds of aquatic animals—fishes, mollusks, crustaceans, etc.—propel themselves. The motion of the medusa is particularly interesting, and the phases of the movement of the umbrella are shown in [Fig. 26]. The propulsion of this mollusk is effected through the alternate contraction and dilation of its umbrella. Ten images per second were sufficient to obtain a pretty complete series of the phases of this motion. These images gain much by being examined in the zoetrope, wherein they reproduce with absolute perfection the aspect of the animal in motion.
FIG. 24.—ARRANGEMENT OF THE AQUARIUM FOR THE STUDY OF
AQUATIC LOCOMOTION.
The [hippocampus], which is otherwise known as the “sea-horse,” affords another interesting example of aquatic locomotion. The principal propeller of this animal is a dorsal fin which vibrates with such rapidity that it is almost invisible, and has an appearance analogous to that of the branches of a tuning fork in motion. With twenty images per second it is seen that this vibration is undulatory. We have before us the successive deviations of the lower, middle, and upper rays of the film. In the present case the undulation takes place from the bottom upwards.