In order to tell how much reduction in size is made by the glass at any given distance, it is possible to look at the original object with one eye and through the reducing glass with the other eye so that by superimposing the two images their length may be compared. When a chart has ruled lines, as co-ordinate lines, it is a simple matter to superimpose the images from the two eyes so that one square of the original equals two, three, or four squares of the image seen through the reducing glass. When the two images are thus superimposed, study can be made of the thickness of lines or other details in the reduced

242 GRAPHIC METHODS

size so that a decision may be had as to whether the drawing will safely stand the proposed reduction without having the lines made heavier.

Fig. 194 shows about the extreme limit of what can be done in the making of pin maps. The original map here was 40 inches wide and 66 inches long. Nevertheless, the map shows up satisfactorily in the greatly reduced size of the half-tone because care was taken to have very wide lines and little detail on the original map. Each of the 19,500 pins of five different colors had a head diameter of V32 inch. It must be remembered, that most of these pins were in the characteristic blue colors commonly associated with the Bell Telephone System, and, because blue is almost impossible to photograph, the pins do not show out as strikingly as they would if other colors were used. The black splotches on the map were caused by the predominance in those areas of pins with dark blue heads. When this pin map was made there was no intention of taking a photograph of it.