Leave the plate over the iodine until it assumes the deep yellow colour bordering on rose. This may occupy from one to three minutes, according to circumstances. Then pass it on to the bromine. Leave it over the bromine until it assumes a blue steel colour. This may occupy from twenty seconds to two minutes, according to circumstances. Then shut the back shutters of the coating box, and draw a yellow blind, formed of three layers of yellow calico, before the window. Return the plate to the iodine, and let it remain from ten to twenty-five seconds, according to cir cumstances.
The white light which falls on the plate while you are observing the colour, produces an effect, for the plate is certainly sensitive at that time, but the second exposure to iodine obliterates that effect, and renders it harmless. If therefore any accident occurs in taking a picture, even should it be exposed to broad daylight in its sensi tive state, the effect of the light may be obliterated by exposing it once more for a few second.s to iodine. The plate need not be cleaned, polished, and sensitized again.
The plate is now ready for the camera, and the sooner the picture is taken the better. It may, however, be kept in a sensitive state for some hours, when proper precautions are taken.
3rd Operation.—To Expose in the Camera.—The exposure in the camera is effected in the usual way. The time of exposure is a matter which can only be determined by trial. The detaiLs in the deepest shadows may always be obtained by sufficient exposure ; it is a mere question of time ; but the lights become blue and solarized by over exposure. It is difficult, therefore, to include successfully very violent contrasts of light and shade, and this should not be attempted as a rule, for the great charm of this process consists in the beauty of the half tones. In landscapes, however, the blue solarization of the sky is an improvement to the picture, and the blue tint generally softens down to a warmer tone on the horizon, which gives a very charming effect.
4th Operation.—To develop the Inur.ge.—Employ a mercury box of any shape, provided with a thermometer. Filter about a table spoonful of mercury into the pan, through a filter with a very small bore, but do not let the filtered mercury fall from a height into the bath, as it oxidizes in passing through the air, and the globules on reuniting become covered with a grey ffim of oxide, which floats on the top, and prevents the vapour from rising so quickly as if the surface were clean.
No picture is visible when you remove the plate from the camera. Place it over the mercury, and heat the mercury with a spirit lamp to a temperature of about 140° Fahrenheit. In a few minut,es the picture will be developed. The time will depend entirely on the
size of the box, a large box requiring more time than a small one. From three minute,s to a quarter of an hour are about the extreme limits of the time required. Remove the plate and examine it now and then by a yellow light. Push the development as far as pos Bible, in order to get out all the details. If you over-develop the picture, or much overheat the mercury, the shadows will be covered with minute specks or pellicles. If the plate has been exposed to too much diffused light, a white film of mercury will be deposited upon it, which will obscure the details, or injure the vigour and inten sity of the blacks. If you under-develop the picture, the lights will want solidity, and the details of the shadows will be defective.
When the image has been developed in the mercury box, the plate may be exposed to moderate daylight ; and the fixing process may be deferred till convenient opportunity.
6th Operation.—To Fix the Picture.—The fixing process includes two operations. The first consists in removing the coating of iodine and bromine, by means of hyposulphite of soda. The second consists in gilding the image by means of a hot solution of eel d'or, which has the effect not only of improving the tone, but also of cementing the image firmly to the plate, so that it cannot be rubbed off with the finger.
Make a solution of pure hyposulphite of soda in distilled water ; the strength is not material, say one ounce hypo to ten ounces water. Filter it carefully into a small upright glass bath, like that used for nitrate of silver. Hold this obliquely in the left hand, and let the plate slide in, with its back on the under side of the bath. We recommend this mode of immersing the plate in the hypo, because if the plate is immersed in a horizontal bath, it sometimes happens that two waves of fluid meet in covering it, and this forms a line across the picture, which cannot afterwards be removed. If the back of the plate is dirty, it contaminates the hypo, and occasions stains on the face ; hence the importance of cleaning the back of the plate. The hypo very quickly removes the steel colour of the iodine, and leaves the mercury, which forms the lights of the picture, adhering to the silvered plate, the black polished silver forming the shadows of the picture. When the steel colour has disappeared, pour the fluid into the funnel, let the plate slide out of the bath into your hand (taking care that the face of the picture does not touch the glass), then holding it horizontally, pour upon it filtered rain-water, from a bottle having a cork into which a small glass tube is inserted. This washing removes the last traces of hyposulphite of soda.