The organisms were placed in a small flat glass tank or vessel D, and were illuminated by the light from an incandescent gas burner. As it was unnecessary to keep the burner alight during the periods when the shutter was closed, Messrs. Bull and Pizon introduced a means of turning the light up and down. This was effected by a small electric magnet, working in synchrony with the clock and controlling the light so that the subject was illuminated only during exposure.
As the studies were prolonged it was essential that the water in which the organisms were placed should be kept fresh and sweet. A glass jar F was introduced to serve as a reservoir, and from this a tube extended to the vessel D. A constant flow of water was thus provided. Its circulation was ensured by another glass tube extending from the vessel P to the waste. The flow of water was controlled to a nicety by means of a tap without the production of bubbles or any other disturbance in the vessel D. In this manner the colony was preserved to the best advantage and in full activity. Some such system of circulation is necessary in all cases where the investigations are to last a long time.
In recent years the Marey Institute has much improved its micro-cinematographic apparatus. The microscope is now carried in a vertical position in front of the support which holds the camera. The camera is fitted with an external bellows which carries a prism at its outer extremity. The prism is brought over the eye-piece of the microscope. The rays of light striking the turning mirror on the base of the microscope are projected upwards through the object side or vessel and then through the eye-piece to the prism. Here the rays are bent at right angles, and are thus directed upon the travelling film in the camera. The camera is driven electrically, the motor being mounted within the box forming the base. Mechanism is introduced whereby the number of photographs per second may be varied within wide limits.
The most interesting feature of this apparatus is the means adopted to enable the worker to follow the movements of the object so that the camera can be stopped when they are of no moment or are not sufficiently near the centre of the picture. A small proportion of the light rays which have passed through the microscope are deflected from the prism mounted upon the eye-piece and thrown into a small view finder beside the camera. Looking into this view finder one can see exactly what is happening upon the stage of the microscope. This novel attachment enables the waste of film to be reduced to almost nothing.