SUBMARINE PHOTOGRAPHY This is an application of photography which, probably on account of the very special appar atus, and the great difficulties encountered, has received but little attention. Attempts have been made from time to time, and with varying success, to photograph the bottom of large rock pools and shallow lagoons, by connecting the camera with a tube of large diameter, pointed downwards through the water. A special raft having a well in its centre, to carry the downward pointing photographic apparatus, has been used with success ; the shadow cast by the raft cuts off all sky reflections. By working at sunrise and an hour or so before sunset, a kind of oblique lighting was obtained which gave very pleasing results, the exposures varying from of a second to two seconds, according to the depth of water, lighting, etc. Green-sensitive orthochro matic plates were found to yield the best results, the greenish yellow tint, caused by the oblique rays of the sun passing through the sea-water, acting as a kind of natural compensating filter.
The actinic value of light decreases very rapidly with increasing depth of water, so that some means of artificial illumination soon becomes necessary. This at once presents many diffi culties, as special apparatus to hold the illu minant becomes necessary, and is rather costly to construct, owing to the perfect fitting necessary to make every part water-tight. Heavy plate
glass must be used for the window of the illu minant box, and the box itself must be well weighted to insure its sinking to the bottom on an even keel. The box must be fairly large, so as to permit of a good sized window, and also a full charge of magnesium powder being used. The best results will be obtained by using two such illuminant boxes, sunk one on each side and slightly in advance of the camera, which also must be enclosed in a water-tight, glass fronted, weighted box. Both the camera shutter release and the ignition of the magnesium flash can be worked by electrical switches. One trouble which may spoil many exposures is the condensation of moisture on the inner surface of the glass front of the box carrying the camera, due to warm moist air within the box. The glass should be very carefully cleaned, the box kept as cool as possible, and if there is great variation between the temperature of the air and the floor of the sea, a small quantity of calcium chloride may be placed in the box at the moment of closing it. This will be found to act as a pre ventive of condensation. F. M-D.