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

are of the same size length than that ot the policeman. The illusion

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

A FEW CAUTIONS

is caused chiefly by the perspective hnes of the drawing which force

one to estimate relative height to a certain extent by the number of

perspective hnes intersected instead of by the actual size of the black

pictures.

Before charts are sent to an engraver to

have plates made for printing it is wise to

have each chart run the gauntlet of a series of

questions, so that the time of the person doing

the checking may be saved and also that the

points more frequently overlooked may be

thoroughly considered in each case. Below are

given a series of questions which may be found

convenient to anyone having charts to prepare.