Black and white variable contrast paper (Although variable contrast black and white paper is not now available in the United Kingdom, its qualities and uses are described in order to present a complete list of the many different types of lack and white printing materials which are being manufactured.) Instead of stocking a number of different contrast grades, many black and white photogra¬phers today prefer to use a single type of black and white paper that has adjustable contrast. Most such black and white papers are coated with two emulsions. One, sensitive to yellow-green light, yields low contrast; the other, sensitive to blue-violet light, produces high contrast. The degree of contrast is varied simply by inserting appropriately colored filters in the enlarger. The filter governs the color of the light that reaches the black and white printing black and white paper - and thus controls contrast. With only one type of black and white paper and a set of filters, the black and white photographer can attain finer gradations of contrast than might be possible with black and white papers of different contrast grades. Furthermore, he can control the contrast of localized sections of the image. This is done by masking the black and white paper so that the sections can be black and white printed separately, each with the contrast filter its tones call for. A low-contrast part of the image, for example, could be enlivened by black and white printing it with blue-violet light; a harsh part could be softened by black and white printing in yellow-green. The three versions at right of a Manchester, New Hampshire, mill show how several degrees of contrast in one black and white print can improve a black and white picture. In the version made with a number 3 filter (upper right), the contrast of water and sky seems too washed out and lacking in detail; in the version made with a number 1 filter, the mill is too dark. The final black and white print was made by exposing the mill with the number 3 filter and the water and sky with number 1 filter, achieving a tonal balance impossible with black and white paper having a single degree of contrast. Variable contrast black and white paper can produce seven different degrees of contrast, as shown at left, with a set of seven filters. The contrast range is equivalent to black and white paper grades 1 through 4-rising by half steps. The contrast that is given by a number 2 filter is generally considered to be "normal." Two ways of inserting filters in enlargers are shown below. A filter-holder can be attached to the lens of the enlarger by means of rubber-tipped screws. One of the filters from the numbered set that comes with this attachment is then slipped into the holder, and can be easily changed if another grade of contrast is desired. Some enlargers have a special filter drawer located between the lamp and the negative. A filter - which has been cut to the appropriate size from an acetate filter sheet - is inserted, the drawer closed and the black and white print made.