Ether, whether prepared from rectified or methylated spirit, is liable to become ozonised, and acid, by exposure to light, in which condition it is unfit for the preparation of collodion. This state can be tested for by agitating it with an alcoholic solution of iodide of potassium. When ozonised, as it is called, the iodine is liberated, and the solution is coloured the characteristic yellow colour of free iodine. The active agent in this case appears not to be ozone, but peroxide of hydrogen.
Eury scope. One of the many names applied to the rectilinear or symmetrical doublet. (See Lens.)
Exposure. Placing any sensitive surface under the action of light, either in the camera or in a printing frame. Of the latter but little need be said, as the result is, in the case of ordinary paper, visible; and in the case of bromide papers for develop-ment, instructions will be found under the heading Bromide Paper. Of camera exposures little can be said here, yet whole volumes might be written without affording much material aid. We may well divide the factors which influence the duration of exposure into five distinct sections: (i) Light, (2) Plate, (3) Subject, (4) Diaphragm, (5) Distance.
(1) Light. In estimating the quality of the light which falls upon thfe subject, we have several points to take into considera-tion. It has been and still is the accredited custom of many operators to judge of the quality of the light by the image as seen on the focussing screen ; that is to say, many operators still employ the method of calculating exposure by means of the optical brightness of the light, and not by the chemical bright-ness - two totally distinct properties of light, between which there is not, unfortunately, any fixed ratio or connection. Other important factors in determining the quality of the light are, climatic or meteorological circumstances, the latitude of the place, and the height of the object above the level of the sea, this last only being taken into account when on mountains, etc. It will be almost impossible to here give full and complete information, but as far as possible within reasonable limits this shall be done. Dr. Holetschek, of Vienna, has reckoned for Vienna and all places of that latitude the chemical light intensity of sunlight and that of a blue sky, these results being embodied in the following tables : -