[Illustration: Fig. 1. Fig. 2.] Fig. 1 represents it placed on a layer of black velvet; fig. 2 as held up to the light. If we term it Positive in the first case, _i. e._ by reflected light, then it is Negative in the second, that is, by transmitted light. The explanation is obvious. Therefore to carry our original definition of Positives and Negatives a little further, we may say, that the former are usually viewed by reflected, and the latter by transmitted, light. All Photographs however cannot be made to represent both Positives and Negatives. In order to possess this capability, it is necessary that a part of the image should be transparent, and the other opaque _but with a bright surface_. These conditions are fulfilled when the Iodide of Silver upon Collodion is employed, in conjunction with a developing agent. Every Collodion picture is to a certain extent both Negative and Positive, and hence the processes for obtaining both varieties of Photographs are substantially the same. Although however the general characters of a Positive and a Negative are similar, there are some points of difference. A surface which appears perfectly opaque when looked down upon, becomes somewhat translucent on being held up to the light; hence, to give the same effect, the deposit of metal in a Negative must be proportionally thicker than in a positive; otherwise the minor details of the image, will be invisible, from not obstructing the light sufficiently. With these preliminary remarks, we are prepared to investigate more closely the _rationale_ of the processes for obtaining Collodion Positives and Negatives. All that refers to paper Positives upon Chloride of Silver will be treated in a subsequent Chapter. SECTION I. _On Collodion Positives._ Collodion Positives are sometimes termed _direct_, because obtained by a single operation. The Chloride of Silver, _acted upon by light alone_, is not adapted to yield direct Positives, the reduced surface being dark and incapable of representing the lights of a picture. Hence a developing agent is necessarily employed, and the Iodide of Silver substituted for the Chloride, as being a more sensitive preparation. Collodion Positives are closely allied in their nature to Daguerreotypes. The difference between the two consists principally in the surface used to sustain the sensitive layer, and the nature of the substance by which the invisible image is developed. In a Collodion Positive the lights are formed by a bright surface of reduced Silver, and the shadows by a black background showing through the transparent portions of the plate. Two main points are to be attended to in the production of these Photographs. First, to obtain an image distinct in every part, _but of comparatively small intensity_.--If the deposit of reduced metal be too thick, the dark background is not seen to a sufficient extent, and the picture in consequence is deficient in shadow.