336 W. MCDOUGALL : These effects are very well illustrated in the study of the ambiguous figures described above. It frequently happens- that when a person looks at one of these figures, e.g., the staircase figure, for the first time, he can see it only in one of its principal forms, even though he is asked to attempt to interpret it in different ways and in spite of the most varied movements and adjustments of the eyes. If then one of the alternative forms, say the broken-wall form, is named, the name, by virtue of a previously established association, calls up the idea of that form, the corresponding upper-level system is excited, the sensory excitation initiated by the figure now strikes at once into this path and the figure is seen in this form. On looking at the figure again a moment later both systems, that corresponding to the staircase and that corre- sponding to the broken wall, are in subexcitement and the excitation of the sensory level may strike into either one, but will then soon pass to the other, and the subject is unable to prevent the alternate appearance of the two forms. When we voluntarily determine the appearance of one or other form of an ambiguous figure, or voluntarily prolong its ap- pearance in one form, we do that not only or chiefly by voluntarily giving to the sense-organ an appropriate setting, but by directly re-enforcing the excitement of the appropriate mental system, just as we voluntarily bring about or re-en- force a contraction of the muscles of a limb by concentrating attention on the idea of the movement to be effected by the contraction. This influence of pre-perception may be very neatly illu- strated by help of figure 2 (p. 335, vol. xi.). If an after-image of this figure is obtained and then allowed to die away while the eyes are completely protected from the light, it can, like other after-images, be revived by allowing light to fall on the closed eyelids. If, at the moment before thus reviving the after-image, one calls up the idea of the figure in any one of its principal forms, the discs will appear in the after-image at the moment of its revival, grouped as they were ideally represented. The excitement of the upper-level system not only co- operates in selecting the object of sensory attention and in determining the mode of its appearance ; it also directly supports the sensations excited by that object as against all other sensations. This is illustrated by those experiments by which it was shown above that the voluntary re-enforce- ment of one colour in the binocular rivalry of two colours, is not wholly indirect reinforcement by way of muscular adjust- ment. It is well illustrated also in the use of a microscope,
