plate, the cover sliding tight over the top, and more than half way down the sides; light is further excluded by means of an outer jacket of tin, which is soldered to the box a little below the centre. The cover thus slides between the case and the jacket, and renders injury to the plates by the entrance of light an impossibility. (Fig. 141.) [Illustration: Fig. 140. A. Loop for finger. B. The knot which prevents the stirrups of silver wire, C C, slipping off the corners of the glass plate. D D. The opposite corners of the glass plate on which the stirrups are placed.] [Illustration: Fig. 141. A A. Tin box, with partitions to hold glass plates, B B. The outer jacket, between which and the box, A, the lid or cover, C, slides.] The sensitive albumenized glass plate is exposed in the camera from fifteen to thirty minutes, and developed (much in the same way as the paper pictures) with one ounce of a saturated [Page 147] solution of gallic acid containing ten or fifteen drops of the sensitizing solution. The plate is usually placed on a levelling stand, and the solution poured on the glass plate; the development is slow, and may be quickened sometimes by the application of heat. The picture is fixed by immersion for a short time in a bath containing one part of a saturated solution of hyposulphite of soda in eight of water. The pictures produced by this process are exquisitely defined, provided always the camera is well focussed, and to assist this operation a magnifying glass may be employed. After removal from the hyposulphite of soda the plate is well washed with water, and being allowed to dry spontaneously, is now ready to print from. _Sixth Experiment. The Collodion on Glass Process._ The glass plates for this, as well as the albumen on glass process, should be cleaned by rubbing them over first with a mixture of Tripoli powder and ammonia, which is washed off under a tap, and the glass being drained is rubbed dry and polished with a clean calico duster kept exclusively for this purpose.