(c) by putting too many facts in one picture so that the eye and mind, when trying to make out any one, are confused by the others.
Illustrations of bad practices in these respects are shown in Figs. 42 to 52. A few specimens of work well arranged for the eye are shown in Figs. 53 to 56.
Good rules to remember are:—
Other things being equal, make distinctions by the clearest method, fit material to the tendency of the eye to see an 'eyeful' at a time (roughly 1½ inch by ½ inch in a book; 1½ ft. by ½ ft. on the blackboard), and let one picture teach only one fact or relation, or such facts and relations as do not interfere in perception.
The general conditions of seating, illumination, paper, and the like are even more important when the eyes are used with numbers than when they are used with words.
Fig. 42.—Try to count the rungs on the ladder, or the shocks in the wagon.
Fig. 43.—How many oars do you see? How many birds? How many fish?