Primuiine (Ger., Primulin; Fr., Primulin; Ital., Primulina). A complex organic dye obtained by the action of sulphur and fuming sulphuric acid on paratoluidin. It has the property of dyeing cotton without any mordant, and is used in the Diazotype process.
Primuiine Process. See Diazotype Printing.
Principal Axis is the straight line which joins the centres of curvature of the spherical surfaces of a lens, or if one surface is plane, the principal axis passes through the centre of curvature of the spherical surface, and perpendicular to the plane surface. A straight line passing through the optical centre, and making an angle with the principal axis, is termed a secondary axis. The centres of curvature are the points from which the arcs of the circles forming the spherical surfaces of the lenses are calculated - that is, the centres of curvature are the centres of circles, of which circles the spherical surfaces of the lens are segments.
Printing. This term is applied to any method by means of which a positive is obtained from a negative, so that a picture is obtained in which to some extent at least the gradations of light and shade are represented as seen in nature. More usually, however, this term includes the production of pictures upon any flexible support, such as paper; and as the other branches of printing, such as bromide paper, opals, and lantern slides by means of which prints are obtained by development, are treated of elsewhere, it is the author's intention to confine this article entirely to what is termed sensitised or albumenised paper printing. If the operator desires to sensitise his own paper, instructions will be found elsewhere, and it is supposed that he has his paper ready for cutting to the desired size. The author would impress upon his readers the necessity for cutting the paper, not to the exact size of the finished print, but rather larger. He recommends the use of an old negative glass for cutting it to shape, as this allows of some margin, and does not require such nice adjustment as when the paper is cut to the exact size. Having the paper ready cut, the next operation is to place it in the printing-frame. This should be done in weak daylight, and the film side of the negative should be placed next to the paper, and one or two thicknesses of blotting paper placed on the back