EXPOSURE, DEVELOPMENT, FIXING, ETC.
Place the plate on the wires (Fig. 160) of the dark slide and insert the same very gently in the camera, drawing out the slide shutter with great care to avoid dust, and after exposure close in the same way : return to the tent and deposit it, still in an upright posi tion. Take sufficient developing solution in the glass, lower the bag, hold the plate as /I1 coating, and pour the solution over, giving a gentle rock to keep it flowing slowly to and fro on the plate. This re quires a little practice. Do not pour on with a splash. Have underneath the plate a bottle with funnel containing a little cot ton-wool. The developer running off the plate is thus filtered ready for use in cases of over-exposure. It is necessary that the exposure be very correct ; this is a matter that can only be learnt by experience, as development cannot be controlled as in dry plate work. Directly the image is fully out (if it is stopped too soon it will be too black ; if allowed to remain too long it will be weak, flat, and foggy), swill and transfer to fixing bath, using another clipper. When the shadows appear quite clear, the ferrotype should be well washed for a min ute or longer. If this is done by pouring over it water out of a jug, do not pour too rapidly and heavily. An over-developed
ferrotype, or one weak and flat, may be improved by adding a drop or two of iodine to the fixing bath. A little of the latter may be taken up in another measure to which this has been added and applied as in developing. After final washing the plate may be dried slowly, face upwards, over a small lamp and varnished, the var nish being applied by flowing over the plate as usual, Ferrotypes can be con trolled but little in development, and it is usually better to take another if greatly in error. Transparent varnish for ferro types is best when bought ready-made ; it is a solution of seed lac in methylated alcohol.
Day-COLLODION PLATES.
What are known as collodion plates are plates of either glass or metal coated with emulsion in which the vehicle used for t he sensitive salts is Collodion instead of gela tine. Collodio-bromide emulsion can be purchased if desired, or it may be made as follows. The chemicals used are silver nitrate, zinc bromide, alcohol, ether, and pyroxyline, and the precautions already referred to must be taken as regards purity. The alcohol must be the pure spirit, not mineralised. The ether may be methylated, but should be of the purified kind, with a specific gravity of