M. Joly has thought to facilitate exhibitions of this kind by making machines that will hold four different images, each of which can be seen by four persons at once.
To attain this result M. Joly has, according to La Nature,* arranged the apparatus vertically, and divides it into two parts: the rear, which contains the mechanism (I. Fig. I 2 I ), and the front which has the eyepieces.
The celluloid bands on which the pictures are taken are fastened end to end; but in order not to increase indefinitely the length to be unrolled, the same arrange ment has been repeated twice, by putting one above the other two identical systems, C and D,formed each of two bands glued end to end (2). To allow all the images to be seen successively the eyepieces are mounted on a vertical slide and can assume two positions, in which they are either in front of C or D. This maneuver is performed very simply by working a lever, A, from the outside (I), which causes them to rise or fall all four at once.
An electric motor, controls, by means of a gear ing, E, one or the other band; the working of the eyepieces by the lever A also acts at the same time on a commutator, G, that so distributes the current that it is always the band in front of which the eyepiece is that is set in motion.
A very simple special arrangement cuts off the cur rent when half the band has passed (that is to say, when one of the subjects has been exhausted); pressing on a button sets the machine in motion again and permits the other subjects to be seen.
To repeat the same series of operations on the other band the eyepieces must be moved. The bands are mounted on bobbins on which they roll and unroll, while keeping a tension that is given once for all at the moment of mounting by a cylindrical spring, B. They pass over a series of cylinders which cause them to take the four positions necessary in order that they should present themselves normally be fore the axis of each eyepiece. Behind them is a drum, T, pierced with longi tudinal opening and containing two in candescent lamps, one on the level of each band. This drum acts as the shut ter intended- to turn on and shut off the light at the intervals necessary to pro duce the illusion sought; it is so regu lated that the passage of the opening over an image coincides exactly with the instant when the latter is found in front of an eyepiece. This arrangement is represented on a larger scale in Fig. 2. It is clear that only one of these two lamps is lighted at a time— the one intended to illuminate the band on whose level the eyepieces are ; when the latter are shifted, the commutator G so moves that the useless lamp is extinguished and the other lighted.
All the manipulation is performed by means of a little arrangement placed behind the case that carries the mechan ism, convenient to the hand of the person who is exhibiting the apparatus, and out of the sight of the public ; it suffices to move a commutator and then to push a button to cause the first band to unroll and present all its images successively to the four eyepieces; as we have said above, the current is cut off automatically after the first band has passed, and when the button is pressed the motor starts again and moves the second band. This first series having been shown, the eyepieces are lowered to the level of the system D and the same series of operations is repeated.
The bands are the same as those of the kinetoscope, and it is easy to change them. This takes a certain time, but the same subjects can be exhibited during:an entire day, and then the exhibitor has time to prepare for the next day by substituting other bands.
Chronophotography has been employed by M. Mach in the study of the growth of plants.
The method of M. Mach does not differ materially from that of M. Marey. It is thus described : * " In both cases the course is to take a certain number of images of the same object under iden tical conditions of light, and of presenting them to the eye, so as to obtain a continuous phenomenon by utilizing the persistence of luminous impres sions upon the retina. The first attempt made by M. Mach gave him excellent results. To render the task as little tedious as possible, he selected a rapid-growing plant, a gourd (Cucurbita pepo), which he photographed twice a day (7 A.M. and 7 P.M.). From June 28th to August i5th, he ob tained ninety-five proofs, reproducing as many different stages of the plant's growth. His arrange ments for maintaining identical conditions during the whole course of his experiments were as fol lows : The vase, containing the seed just beginning to germinate, was placed on a firm stand before a background of solid color; an arc-lamp placed above supplied the necessary light. On commencing the experiments, the blinds were drawn to ren der the place perfectly dark, and to give full effect to the electric light. The time of exposure de pended on the intensity of the light ; but being once determined, was:adhered to, without variation, through the whole course of the experiments.