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.

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. courtesy or the Grolier society Therc arc fcw pcoplc who will beheve until Fig. 254. An Optical lUu- ^^ ^^^^ measurements that the figure

The white square appears larger of the girl lU Fig. 256 IS really of greater than the black one, yet the two -i ,-• ,i ,i ■ j? . i t mi -n