The printing is conducted by the light of gas, or of a camphine or moderator lamp; diffused daylight would be too powerful. The employment of a concave reflector, which may be purchased for a few shillings, ensures parallelism of rays, and is a great improvement. The lamp is placed in the focus of the mirror, which may at once be ascertained by moving it backwards and forwards until an evenly illuminated circle is thrown upon a white screen held in front. This in fact is one of the disadvantages of printing by a naked flame--that the light falls most powerfully upon the central part, and less so upon the edges, of the Negative. The picture must be exposed for a longer or shorter time (about ten seconds will be an average) according to its behaviour during development (see p, 224); this process, as well as the fixing, is conducted in the same manner as for Collodion pictures generally. Some adopt the plan of whitening by Corrosive Sublimate, and again blackening by dilute Ammonia, as an improvement to the colour of the dark shadows (see p. 113). If this mode of printing upon Collodion be conducted with care, the Negative being separated from the film by the smallest interval only, the loss of distinctness in outline will scarcely be perceived. Stereoscopic transparencies may also be printed by the dry Collodion process described in Chapter VI., or by the Collodio-Albumen process. Mr. Llewellyn recommends the employment of a solution of Oxymel, so dilute that the plate becomes nearly dry, and may be laid in contact with the Negative without fear of injury (see the footnote at page 302). CHAPTER V. CLASSIFICATION OF CAUSES OF FAILURE IN THE COLLODION PROCESS. Section I.--Imperfections in Collodion Photographs. Section II.--Imperfections in Paper Positives. SECTION I. _Imperfections in Negative and Positive Collodion Photographs._ The following may be mentioned:--fogging--spots-- markings, etc. CAUSES OF FOGGING OF COLLODION PLATES. 1. _Over-exposure of the Plate._--This is likely to happen when using the full aperture of a double combination lens for distant objects brightly illuminated, the Collodion being highly sensitive. Also from the film being very blue and transparent, with too little Iodide of Silver (p. 114).