An obvious defect in a daguerreotype picture placed in filtered rain-water for a second only, and then immersed in one of the fixers just named until the yellow colour had quite dis appeared ; warm distilled, boiled, or filtered rain-water was then allowed to run in a stream over the plate in order to wash it. The shadows were represented by the polished surface of the silver, and the lights by the adhering and very delicate film of mercury, which, if fingered in any way, would be wiped off. Therefore, in order to preserve the pictures, they were placed under glass and the air excluded. In some cases the picture was treated with a solution of gold and sodium hyposulphite, which brought out the details with greater force and brilliancy. This idea was originated in 1840 by Pizeau, of Paris, who used a solution of 7 grs. of gold chloride in to oz. of distilled water, this being mixed with a solution of 3o grs. of sodium hyposulphite in 4 oz. of water.
The expense of the silvered plates was a taken in an ordinary camera was that the image was reversed, exactly as in a ferrotype (or tin type) portrait. To obviate this, it was neces sary to reverse the image by means of a mirror attached to the lens, thus increasing the already lengthy exposure by about one-third.
The fourth process was that of development, or " mercurialisation " of the image on the exposed plate. The latter was taken into the dark-room and placed in a dark box C in such a way that the surface was suspended over a saucer of mercury heated to a temperature of about 14o° P. (6o° C.). The box had under its bottom a lamp br, which heated a dish of mercury in which was a thermometer N. I he plate P, as removed from the camera, was held in a grooved blackboard Q, where it could be viewed through the glass panel R. The lid of
the box is shown by s. The fumes of the mer cury " developed " the image in the course of about twenty minutes. The final operation consisted in removing the unused iodine from the plate of silver in order to prevent the further action of light. A saturated solution of common salt was first used, and later a weak solution of sodium hyposulphite. The developed plate was great drawback to the daguerreotype process. As late as 1853, the price charged for a quarter-plate daguerreotype portrait was fifty shillings, and for a half-plate eighty shillings. It was the custom to " improve " daguerreotype pictures by colouring them. Colours ground extremely fine were used and dusted on dry with a fine camel-hair brush, the process needing great care, as it was almost impossible to remove any of the colour applied. When the colours were on they were breathed upon to make them adhere. Claudet's method was to mix the colours with alcohol, and apply cautiously with a soft brush, and to dust on dry colours if the liquid colours were not dark enough. The colours chiefly used were gold, carmine, chrome yellow, and ultramarine, by combining which any desired tint could be obtained.
Major-General Waterhouse has found that daguerreotype plates can be developed with a wet collodion (physical) developer to give a positive image as usual, or with an alkaline developer, or with ferrous oxalate, to give a negative image. If before exposure the plates are treated with an alcoholic solution of erythrosine, they show sensitiveness to the less refrangible end of the spectrum. Copper plates sensitised with iodine and bromine yield images if exposed and developed with ferrous oxalate or an alkaline developer, and the results are fairly certain.