Page:Graphic methods for presenting facts (1914).djvu/378

 the judgment can be trained very greatly if it is kept in mind to examine every curve chart which comes to one's attention to see whether the vertical and horizontal scales have been so selected that the chart gives a fair representation of the facts.

Courtesy of the Grolier Society

Fig. 252. An Optical Illusion

The black line at the left appears longer than the one at the right. The two lines are, however, of the same length

Courtesy of the Grolier Society

Fig. 253. An Optical Illusion

The left-hand arrangement looks wider than it is high while the right-hand arrangement looks higher than it is wide. On each side the height is the same as the width

The English language has so many words with double meaning and so many words for which the shades of meaning are rather indistinct that there are really many more chances of false impression from the written or spoken language than there are from the data expressed in graphic form. Nevertheless, a few examples of optical illusions are shown here so that the reader may have some idea of those peculiar things which may enter in to cause strange impressions if charts do not receive some degree of preliminary care and final inspection. Though many of the effects seen in Fig. 252, Fig. 253, Fig. 254, Fig. 255, and Fig. 256 are not likely to appear in ordinary chart work, they may notwithstanding cause difficulty in some kinds of very large wall exhibits. Fig. 254, in particular, shows an effect which is to be avoided where large quantities of black ink are used. In a recent series of charts comparisons were made between different white squares, surrounded in each case by a black border practically as wide as the square at the center. It is not likely that a reader seeing a series of black squares with white centers of different size would be able to judge correctly the relative size of the white squares at the center.

Courtesy of the Grolier Society

Fig. 254. An Optical Illusion

The white square appears larger than the black one, yet the two are of the same size

Fig. 256 shows some of the difficulties which may be encountered if an attempt is made to present data by comparing the relative heights of pictures of the human form. There are few people who will believe until they make measurements that the figure of the girl in Fig. 256 is really of greater length than that of the policeman. The illusion