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26 THE ADVANCE OF PHOTOGRAPHY
And now a peculiar action of the fluids takes place, for the ether in the collodion film repels like fat the aqueous solution of silver, and a steady movement of the plate in the bath is necessary in order to make the solution adhere to the film.
This mechanical operation is accompanied simultaneously by a chemical change. The iodides and bromides in the film undergo a double decomposition with the nitrate of silver of the bath, with formation of iodide and bromide of silver and nitrates of the metals previously combined with the iodine and bromine. The iodide and bromide of silver colour the film yellow ; and now the plate is ready to be exposed in the camera.
When the exposure has been accomplished the sensitive plate is brought back into the dark room. For the purpose of transporting and exposing the plate, which must of course be guarded very carefully from the daylight, a flat frame called the dark slide is employed. This dark slide differs slightly from those now used with dry plates, as will be seen from the following description and fig. 7. At the back of this slide is a door D opening upon hinges, and in front there is a sliding shutter H. In the corners are fixed four silver wires d, d, d, d, upon which the plate rests with its prepared side downwards, being held steadily in its place by a spring / /, fastened to the door D. The plate is carried in the dark slide to the camera and substituted for the movable ground-glass screen upon which the image of the object has been previously focussed. After the exposure the dark slide is removed from the camera and taken back to the dark room.
And now follows one of the most important operations, the development of the picture. Upon the plate there is as yet no trace of a picture visible. The action of the light produces a peculiar change of the iodide of silver which forms the important constituent of the film ; this iodide acquires through the light the property of attract