Page:Graphic methods for presenting facts (1914).djvu/250

 classes of information, it is impracticable to use the cloth-head tacks because the heads are so large that the tacks touch each other in all thickly populated regions.

Photographs taken of a map containing tall pins or tacks give an inaccurate effect, since the angle of the lens causes the head of the tack to appear at one side of the point in which the tack itself is located. Thus, in a photograph of a map of the United States the head of a long pin or tack set at Providence, R. I., might well show near Boston, Mass. There is no way of avoiding this error if projecting pins and tacks are used. The only safe plan is to use the spherical pin head which is in contact with the map surface itself. Photographic views of a sphere are the same from all directions, thus causing all pins to appear exactly the same size and shape on the photograph. Photographs taken of a large map with flat-headed tacks show the heads in the center of the picture as circles, while the heads towards the edge of the picture look of much smaller size, because they appear flattened out as ellipses. The visual effect for points toward the edge of the map is lessened when the tacks are flat headed, and an unnecessary visual error is brought into the picture.

A very cheap yet satisfactory mounting for a wall map to be used with glass-head map pins is made with three or more layers of corrugated straw-board. The straw-board used should be about 3/16-inch thick with a facing on either side of the corrugated portion. Three thicknesses of straw-board are sufficient to give strength for any map up to one yard long. The two outside layers of straw-board should be so arranged that the corrugations will run with the length of the map, thus giving the greater strength in that direction. The middle layer should have the ribs running crosswise of the map so that the map mounting will be safeguarded from bending in either of the two different directions. Where very large wall maps are desired, six or more layers of straw-board may be used to give sufficient strength. If single sheets of straw-board cannot be found as large as the map itself, the map mounting can easily be built up of small sheets of straw-board, provided the joints in the straw-board are so placed that they will not be over each other to weaken the finished structure.

The straw-board backing for a map to be used with pins is not the best obtainable if the pins must be put in and taken out frequently. When, however, the pins are to be placed in the map and left there, the straw-board is just as satisfactory as any other backing for a