A Manual Of Photography

ON THE PRODUCTION OR POSITIVE PHOTOGRAPHS. 297 then, after being well dried, steeped in the following solution :— Water fourteen parts, with one part of a compound formed of two parts of muriate of ammonia, two parts of bromide of sodium, and one of chloride of strontium. The paper dried again is passed into a very weak solution of nitrate of silver. There is thus formed, by double decomposition, a chloride and bromide of silver, which is made to turn blade by exposing the paper to the light for about half an hour. To use this paper, it is steeped in a very weak solution of the iodide of sodium, and placed, quite wet, into the camera obscura, at the proper focus. In fine weather, M. Verignon states, the effect is produced in twelve minutes. I have, however, never produced a good picture by this process in less than thirty minutes. A great objection to this mode of preparation is the very rapid dete- rioration of the paper : every day it will become less and less sensitive to light, and at the end of a fortnight it is useless. The papers recommended for use in the former pages have the advantage of keeping well, provided ordinary care is taken with them. It is necessary to exclude them from the light, to keep them very dry, and, as much as possible, they should be pro- tected from the action of the air. I have kept papers, prepared with the muriate of ammonia, baryta, and strontia, tor twelve months, and have found them but very little impaired. Dr. Schafhaeutl allows paper prepared in the way mentioned at a former page to darken in a bright sun light. It is then Impressed Spectrum. Visible Spectrum. 78.