The sensitive plate or film is a very wonderful thing. The essential part of it consists of a thin film of gelatine containing certain salts of silver. When light strikes this compound of gelatine and silver it upsets it in such a peculiar way that, although not altered in appearance, when placed in various chemical solutions the silver salts are reduced to a metallic state. It is this fine deposit of metallic silver which forms the negative image. When we know that light striking the plate for an infinitesimal period of time is able to affect it in this way we can realize the immense importance of care in loading our dark slides or changing boxes by means of a safe light. Generally speaking, all ordinary plates, even the most rapid ones, are but slightly sensitive to the red rays contained in white light and therefore a red light is used in the dark-room for examining and developing the plates. For special purposes, however, plates are made sensitive to red light and are then known as " panchromatic." Such plates require the very greatest care and should be developed in the dark or by means of a special green light which only passes that portion of the spectrum to which they are most insensitive. Other plates are called Isochromatic, Chromatic or Orthochromatic, and these, while being more sensitive to yellow light than the ordinary brands, can safely be developed by a good ruby light.
ORDINARY V. ORTHOCHROMATIC PLATES.
During recent years there has been a decided tendency to use colour-sensitive plates and it may seem late in the day to advocate the ordinary plates for general photographic work. While we are quite prepared to admit the claims of the advocates of colour-sensitive plates for special purposes, such as copying pictures and cloud photography, we are disposed to think that in endeavouring to truthfully portray colour values they lose the greater and more important truth of colour contrast. In landscape work, particularly when a screen is used, there is a pettiness and spottiness in the resulting picture ; quite apart from its false sky which may in tone represent the dark blue of the vault of heaven,
Bockleton Church. Snapshot, taken with the Sinclair "Una." but entirely opposes the fact that it is the source of light. Such orthochromatic and panchromatic photographs, although possibly admirably and exactly portraying the details of natural phenomena, yet entirely fail to stir the feelings, because they do not record the impressions of air and space.
For general work, including architecture and portraiture, we advocate the fastest possible plates. An extended period of photographic work, during which many thousands of plates or films have passed through our hands, shows conclusively that the great majority of results are spoilt by under-exposure. For our part we look forward to the time when the speed of plates will be greatly augmented. When a snap-shot picture can be taken by lamp light with an f/6 or f/8 lens we shall begin to be satisfied, but not before that desirable era.
FAST OR SLOW PLATES.
A fast plate, excepting for copying line subjects, has no drawback that we have been able to discover, although writers in text books, perhaps stimulated by the remarks of plate-makers, have consistently advocated plates of low speed. A fast plate has a longer range of gradation, and, generally speaking, gives more harmonious prints and enlargements, and providing care is taken to have proper dark-room illumination we should strongly recommend their use.
For copying line subjects a " process plate " which tends to give hard results is desirable and, of course, orthochromatic or panchromatic plates with proper screens must be used for copying pictures or other works of art.
For telephotography it is often necessary to cut out blue haze and then panchromatic plates with a proper screen will give the result demanded.
All the remarks we have made concerning plates apply equally to films, but in the claims between these two vehicles for carrying the sensitive emulsion we recommend plates where their use is at all possible. We are not unmindful of the value of films for many purposes, and their general quality is now very high, yet for the very best results plates should be used. There seems to be some antipathy between the celluloid film and the gelatine emulsion, which conduces to the deterioration of the latter in a most unaccountable manner. An emulsion always seems slower when on a celluloid support ; and, generally speaking, for scientific work, either in hot or very cold climates we have had occasion to say that " weight for weight plates will give better results." Still the
PLATES OR FILMS.
FILMS FOR THE TROPICS.
weight of plates is frequently an effectual bar to their use. When such is the case, and when films must be used, it is essential when taking them to hot or moist climates to properly pack them in tin cases with some damp-absorbing material. Moreover, they should be kept as little a time in the camera as is possible and in no case should they remain in the camera over night.
For lantern slides and transparencies special plates are made by all makers, but if it is desired to make a transparency from which to make an enlarged negative, then such lantern plates should not be used unless the original negative is a particularly thin one. For such transparencies from good negatives we recommend an " ordinary " plate of any good brand, and it should preferably be backed. The transparency to aim for, is one thin and full of detail, and absolutely free from fog. When laid film downwards on a sheet of white paper it will show all the details of the picture excepting in the very highest lights. From such a positive a negative of any character may be made on a good ordinary plate.
PLATES FOR TRANSPARENCIES WHEN ENLARGING.
The best results are most easily secured by what are termed " backed " plates. Backed plates have the glass side covered with some pigment and the object is to absorb light which passes through the sensitive film and which otherwise being reflected back again from the glass surface impairs the perfection of the result, by forming a halo round any strong high-light. Backed plates are always desirable even for making lantern slides, and we never take a negative on any other.