While the rivers run perfectly clear and bright under the ice in the winter, in summer the turbid water of nearly all our large streams introduces another difficulty, and photographic operation must sometimes be deferred for weeks, unless the rain barrels be full or enough ice be found in the ice-house, over and above the domestic needs, to serve. [Sidenote: EFFECT OF COLD ON EMULSIONS] It seems certain that the speed of the sensitive emulsions with which the films are covered is reduced in very cold weather. To determine whether or not this was so, the following experiments were resorted to. The camera was brought out of the house half an hour before noon, at 50° below zero, and an exposure made immediately. Then the camera was left in position for an hour and another exposure made. There was little difference in the strength of the negatives, and what difference there was seemed in favour of the second exposure. Evidently, if the emulsion had slowed, the shutter had slowed also; so opportunity was awaited to make a more decisive test. When there remained but one exposure on a roll of film, the camera was set outdoors at a temperature of 55° below zero and left for an hour. Then an exposure was made and the film wound up and withdrawn; while a new film, just brought from the house, was as quickly as possible inserted in its place and a second exposure made. The latter was appreciably stronger. Even this test is, of course, not entirely conclusive; one would have to be quite sure that the emulsions were identical; but it confirms the writer's impression that extreme cold slows the film. It would be an easy matter for the manufacturers to settle this point beyond question in a modern laboratory, and it is certainly worth doing.