The advance of photography : its history and modern applications (1911)

160 THE ADVANCE OF PHOTOGRAPHY particular make of plates or papers, the object of this chapter being to give such information as may be of general interest to the worker, and at the same time make clear some of the more important facts in this branch of the work. Backed Plates: Halation. — Sometimes, when photographing the interior of a church or other building in which dark masses of shadow necessitate a long exposure, it is found on developing the plate that the high lights have spread out considerably beyond their proper boundaries, and have consequently blotted out much of the surrounding detail. The effect so produced is called Halation. This is due to the fact that the very bright light has not been completely absorbed by the film, but some has been able to pass through it and has then been reflected from the back of the plate at such an angle that it reaches the under surface of the film, and, owing to the length of the exposure, this produces such an effect on the chemicals contained, that it helps to blot out the image which should be produced by the direct action of the light. There are means by which such a plate can be reduced locally by the application of some reducing agent, or the high lights may be reduced to a certain extent by placing the negative in alcohol after it has been fixed, and gently rubbing the parts with either a very soft piece of linen or a plug of cotton wool. It is far better, however, to avoid the error than to try to remedy it, and this can be done by employing what are known as backed plates. Such plates can be prepared at home if desired, but it is better to buy them already backed, as the increase in cost is not great and the chance of failure less. Whatever the nature of the colouring matter used in the backing, which is placed as a coating on the back of the sensitive plate, it must be such that it absorbs the light which passes through the film, and so prevents its reflection