NEGATIVE, PERFECT The term " perfect negative " is frequently met with in photographic literature ; but a negative that is perfect for one printing process may be quite unsuitable for another one. A nice-looking, clean negative, say, one developed with hydroquinone, will often give a print far inferior to one sometimes obtained from a yellow-stained pyro-develbped negative. Of the good average negative, R. Child Bayley says that, placed film side down, upon a sheet of white paper, and looked at from above, the edges on which the light did not fall in the camera ought to be almost clear ; a slight trace of greyness is unavoidable, unless a backed plate is used. There must be an appreciable quantity of greyness in the very deepest of the shadows, or the plate has not had sufficient exposure ; if there is too much the plate has been over-exposed. The highest lights, the most opaque portions, are tested by placing the film in contact with printed matter ; the print must be considerably darkened as compared with the lighter parts of the negative, but it should be possible to read it through even the most opaque portions without difficulty ; other wise, whether the plate was over-exposed, correctly exposed, or under-exposed, it has been over-developed.
At one time the wet-collodion negative was thought to give the effect to be aimed at in the dry plate ; but at the present time the Hurter and Driffield dictum, that the negative should be judged by its printing qualities alone, is generally accepted.
In process work, various classes of negatives are used. They may be in " half-tone," " grain," or in " continuous tone." The last-named resembles an ordinary negative, with gradated tones, not broken up into dot or grain, but must be of very good quality, with full range from almost clean glass in the shadows to perfect opacity in the lights. Such a negative for photogravure or collotype should not be hard, but somewhat thin and full of detail, with a uniform gradation throughout.