Fig. 3 therefore is a Positive copy of Fig. 1, obtained by means of a Negative. By the first operation the tints are reversed; by the second, being reversed again, they are made to correspond to the original. The possession of a Negative therefore enables us to obtain Positive copies of the object, indefinite in number and all precisely similar in appearance. This capability of multiplying impressions is of the utmost importance, and has rendered the production of good Negative Photographs of greater consequence than any other branch of the Art.
The same Photograph may often be made to show either as a Positive or as a Negative. For instance, supposing a piece of silver-leaf to be cut into the shape of a cross and pasted on a square of glass, the appearance presented by it would vary under different circumstances.
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.