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operation the lighting supply and the camera except at the moment of taking each photogram. Microscopic cultures are therefore not subjected to the action of the light except at the actual moment of making the pictures. To be able to do this, a controlling apparatus is required, a chronometer, Kl which with the assistance of relays, W to begin with, operates the light supply, and then when the latter has reached its maximum intensity, operates a small motor W which makes the camera handle turn. Again, it is possible to slow down the picture-making in relation to the degree of strength of the light, which is of considerable importance when it comes to using lamps which light slowly. This slow motion picture camera permits the use of intervals of 15, 20, 30 and 40 seconds, of 1, 2, 5, 10 minutes and of several hours.
The microscope is a Leitz with double observation device. A lens is in the eye-rest ^1 and a prism directs 3 per cent of the light towards the observer's eye and 97 per cent towards the film, so that the material can be observed even during the taking of the pictures, which permits of regulating and, if desired, improving the enlargement. The eye-rest of the observation device shows the image through a net of small lines, which allow the field to be regulated and corresponds to the superficies of each photogram. The field is regulated by means of the eye-piece.
For the study of cell structures of a particularly delicate character, such as the mithocondra, the nature of which in cell life is still unknown, illumination against a dark background is particularly important, because it allows the use of condensers with a completely dark background which have the greatest illuminating power, like Zeiss 's cardoid condensers or Leitz' mirror condensers. In order to take cinema pictures of small drops on a dark background, condensers with long focal extension are indispensable.
In order to observe an action of intervention in cell life, such as the injection of medicated substances into the cells or in their vicinity, or for a mechanical action on separate cells or a group of cells, a micromanipulator installed in the microscope is necessary. In the experiments to which reference will be made, we used a Chamber micro-manipulator made specially for us by the Leitz people.
In micro-cinematography, only powerful electric lamps can be used as light supply, because the exposures for the pictures at normal frequencies, and still more so in the case of those made at higher frequencies (in speeded-up picture-making) are extremely brief.