[10] In order to obtain the effects described in the various
illustrations it is necessary in several cases to regard the figures
for a considerable time and with close attention. The reader is
requested not to give up in case the first attempt to secure the
effect is not successful, but to continue the effort for a reasonable
period. Individuals differ considerably in the readiness with which
they obtain such effects; in some cases, such devices as holding the
diagrams inverted, or at an angle, or viewing them with the eyes half
closed, are helpful.