PROPORTIONAL SCALES AND RULES Various forms of proportional scales or rules have been suggested for the use of photographers and process workers, principally with the object of calculating exposures and the proportions of reductions and enlargements. J. A. C. Bran fill's proportional rule, A, has three members, A, B, and c, which form a triangle graduated on three sides. When c is slid along A, any triangle formed will be similar to the original one, and all its sides will be proportional. This enables a fourth proportional to three known quantities to be found. Thus, if it is required to find x 6 by the rule, place c to 14 on A, set B to 6 on c, slide C to 5 on A, when B will be found to cut C at the dimension required. A shows the positions of the members for this calculation.
Another form of proportional rule, but with a definite purpose, is the scalometer of W. Laurence Emmett, for determining which originals are in the same scale of proportion, so that they may be photographed together on the same plate. This instrument is illustrated in the article under the heading " Focusing," and is used as there described. By its means the operator groups up on his copyboard all originals which bear the same proportional number. By a slight variation of the method enlargements may be similarly dealt with. By means of a Printed scale sent out with the instrument, it is possible to mark off the base of camera and copying stand with numbers corresponding to those on the•hastrument, so that the camera can be instantly set to the proportion number marked on the original, no focusing being then required. A rule and system having the same object was designed and introduced in America by A. Fruwirth, who also applies scales to the base of the camera. Geo. H. Benedict, of Chicago, has a system worked with a chart of curves. All these methods have the same
object, that of setting the camera to the pro portion of the original without the necessity of focusing.
Various rules and scales have been devised for determining the second dimension of a reduction or enlargement, one dimension being given. The sizeometer, B, is a good example of a rule for this purpose. The slotted rule is laid on the diagonal of the photograph, and the right angle rule is then brought down to the dimension of one side of the picture. The second dimension is then read off on the other limb. Carl Norman's proportional rule consists of a strip of stretched elastic bearing a numbered scale, the elastic being mounted between two clamps, so that one of them can be made to slide along and thus alter the length of the elastic ribbon, the scale on it altering proportionally. Thus, the elastic is stretched until a number on it corresponds to the required dimension of the original. Then without altering the elastic the other dimension of the original is measured, and the result is the second dimension.
A further series of proportional scales are those used for measuring the surface area of blocks. Branfill's chart (it consists of a series of curved lines) is a good example. The block to be measured is placed up to the top and left-hand border line, and the required dimension read off on the curved line adjoining the lower right hand corner of the block. Geo. H. Benedict, of Chicago, is the author of a similar chart. Another form of chart for the same purpose is ruled off into i-in. squares, and the figures denoting the square measurement are placed progressively in these squares.