This they do to some extent in point of intensity, for, in spite of the diminished excitability of the centres, the mode of stimulation which occurs in sleep may, as I have hinted, involve an energetic cerebral action.

And, however this be, it is plain that the image will gain a preternatural force through the greatly narrowed range of attention.

When the mind of the sleeper is wholly possessed by an image or group of images, and the attention kept tied down to these, there is a maximum reinforcement of the images.

But this is not all.

When the attention is thus held captive by the image, it approximates in character to an external impression in another way.