The advance of photography : its history and modern applications (1911)

HISTORICAL SURVEY 31 them, they are immersed in a dilute solution of chloride of gold. This process is called toning. Toning of Prints. — In this operation gold is precipitated on the picture, giving it a bluish shade ; and now the tone of the picture is not essentially altered by hypo-sulphite of soda. The picture thus produced consists partly of gold, partly of silver, in a finely divided state, and only requires to be thoroughly washed in order to become perfectly permanent. If this washing is omitted, small particles of hypo-sulphite remain behind, which decompose and form on the picture yellow sulphide of silver. This accounts for the circumstance that the pictures of an earlier period, when from ignorance of this fact this thorough washing was neglected, so often became faded and yellow. It is surprising what a small amount of silver and gold is required to give an intense colour to a whole sheet of paper. For in a sheet of perfectly blackened paper 44 x 47 centimetres there is only 0*15 gramme of silver, and in a photograph of the same size only 0075 gramme ; in a carte de visite but 0*002 gramme. It must be here remarked that prints bleach a little in the fixing process ; and hence the photographer usually lets the prints become darker than they are to remain. Thus even the printing process requires a practised eye, simple as it may appear. Vignetting. — In certain cases tricks of art were employed then as now to produce agreeable effects, and among these is that of vignetting. Our readers are no doubt well acquainted with portraits on a white ground, the outlines of which gradually become confounded with the ground tint of the picture . This effect is produced in a very simple manner by placing what is called a mask over the copying frame. This mask is a piece of metal or cardboard (fig. 9) in which an oval hole b is cut. This is placed on the printing frame