INSTANTANEOUS PICTURES. Photographs obtained in a frac tional part of a second of time are said to be instantaneous. There is no particular process more sensitive than any in common use by which instantaneous pictures may be taken. Either the ordinary collodion process in its most efficient working state, or the daguerre otype process, is employed for this class of pictures. The instan taneity of the exposure is got by increasing the intensity of light in the image by using a lens with large aperture, which must, of course, be provided with the means of opening and shutting it in stantaneously. For this latter purpose many different plans have have been suggested. It is evident that the space to be uncovered and closed should be the smallest through which all the light passes ; hence the absurdity of spring shutters for the dark slide, for the picture is the largest space on which light is incident. The proper place for the instantaneous cap, as it is called, is in front of the front lens.
According to some recent experiments of Professor Zautedeschi, of Padua, it appears that the presence of the atmosphere in contact with the sensitive collodion film retards the action of light, and that sensitive iodide of silver is darkened much more rapidly " in vacuo."
It seems probable, therefore, that the chemistry of the process remaining the same a shorter exposure might suffice if the camera could be exhausted of air after the insertion of the plate. This experiment might be tried without any great difficulty or expense, and, if successful, it would be a great step in photography. It seems probable enough that a quantity of air entangled with the other chemicals in the sensitive film might interfere with their mutual reactions under the influence of light, while the atmospheric pressure of 151bs to every square inch of ffim might render it more compact and less sensitive. Besides, should any gas be given off during the decomposition of any of the chemicals by light, this would readily escape and diffuse itself in vacuo, instead of remaining to interfere with farther decomposition. Both theory and experiment seem, therefore, to be in favour of exhausting the camera of air in the instantaneous processes.