=Physical Illustrations of Attention.=--To furnish a physical illustration of the working of attention, some writers describe the stream of our conscious life as presenting a series of waves, the successive waves representing the impressions or ideas upon which attention is focused at successive moments. When attention is in a diffuse state, consciousness is likened to a comparatively level stream. The focusing of attention upon particular impressions and thus making them stand out as distinct states of consciousness is said to break the surface of the stream into waves. This may be illustrated as follows: [Illustration: FIG. 1--Consciousness in a state of passive reverie. FIG. 2--Active consciousness. Attention focussed on the definite experiences _a, b, c, d, e, f, g_.] By others, consciousness is described as a field of vision, in which the centre of vision represents the focal point of attention. For instance, if the student intent upon his problem in analysis does not notice the flickering light, the playing of the piano, or the smell of the burning meat breaking in upon him, it is because this problem occupies the centre of the attentive field. The other impressions, on the contrary, lie so far on the outside of the field that they fail to stand out in consciousness. This may be represented by the following diagram: [Illustration: P represents the problem on which attention is fixed. A, B, C, D, E, represent impressions which, though stimulating the organism, do not attract definite attention.] It must be understood, however, that these are merely mechanical devices to illustrate the fact that when the mind selects, or attends to, any impression, this impression is made to stand out clearly as an object in consciousness; or, in other words, the particular impression becomes a clear-cut and definite experience. [Illustration: Probable adjusting of nerve ends during active attention]