The heavy black type with much greater spread horizontally than vertically tends to overshadow the curve itself and causes for the curve a more distinctly horizontal impression than would otherwise be obtained. A person reading charts must take great care that he does not give too much weight to the actual appearance of the curve on the page, instead of basing his conclusions on the percentage increase or decrease as judged from the figures of the vertical scale. The proper choice of scales for curve plotting is largely a matter of judgment, and

GRAPHIC METHODS

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

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.

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.