POSITION OF THE LIGHT FILTER.
Considerable controversy has taken place upon the best position for the light filter, and a few words on this subject may prove of service. the coloured glass forming the screen is optically worked, that is, made perfectly flat upon both sides, its position, from a practical point of view, is unimportant ; although it must not be forgotten that the nearer the screen is to the plate the less light is lost. This applies with almost equal force when a filter is used which is ex tremely thin, such as a dyed sheet of collodion or gelatine. Such filters, how ever, especially when home made, are often a little cloudy, and apt to interfere with proper definition, there fore the nearer they are to the plate the better. Again, it should be understood that the unevenness of the surface is more apparent the smaller the bundle of rays which has to pass through it. So that if the filter is placed in the diaphragm opening, where the bundle of rays is very small, the uneven ness is most harmful, whilst _as it is removed from this towards the plate, any defect becomes less noticeable. Optically worked filters are usually fitted in a rim . and made to screw behind the lens as in Fig. 597, but home-made filters should be fixed either in contact with the plate or which could never be substantiated. Some workers pride themselves upon using these plates for every description of work, but it will hardly be necessary to point out to the practical photo grapher the disadvantages of such a prac tice. The orthoehromatic plate is in the
nature of a special appliance, and should be reserved for special use. The inferior latitude permissible in the exposure, de velopment, and gradation of the plates, and the increased precautions necessary in their manipulation, render them un suitable for general use, important as they are in their particular sphere. For example, a large number of landscapes that are brilliantly lighted will photograph equally well upon an ordinary dry plate, and there will be scarcely any advantage in the use of orthochromatic plates. Most exposures made near the middle of the day upon ordinary subjects, such as are attempted by the general run of hand camera workers, will come under this heading. It is when the preservation of contrast in certain coloured portions of the subject is essential that the use of orthochromatic plates is imperative. Also, bearing in mind that these plates possess a superior sensitiveness to rays of certain colours, when photographing in light of that colour they will be of assistance in shortening the exposure. To secure cloud negatives, the plates are often extremely useful when used in con junction with a screen. In ordinary land scape work the use of a screen. or light filter will aid in bringing out the clouds in the sky, but they can generally be more successfully added afterwards, and are usually obtainable only at the expense of the gradations in the foreground.