Page:Popular Science Monthly Volume 18.djvu/74

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Together these alarms form an excellent protection against the two most common dangers to which buildings and their contents are exposed. The addition of the call-bell system, now so common in hotels, business houses, and the better class of residences, completes an electrical equipment that leaves little to be desired in the way of security and convenience.



THERE are great differences in the power of forming pictures of objects in the mind's eye; in other words, of visualizing them. In some persons the faculty of perceiving these images is so feeble that they hardly visualize at all, and they supplement their deficiency chiefly by memories of muscular strain, of gesture, and of posture, and partly by memories of touch; recalling objects in the same way as those who were blind from their birth. Other persons perceive past scenes with a distinctness and an appearance of reality that differ little from actual vision. Between these wide extremes I have met with a mass of intermediate cases extending in an unbroken series.

"We must establish clearly what we are talking about by contrasting in general terms the physiological basis of sight itself with that of sight-memory. Let us put the question to ourselves," "What should we expect to be the effect on our nervous system, first, when a sudden light is flashed on the eye, and, secondly, when we recall an image of that flash?" If we had means of watching what took place, we should no doubt be aware, in the first case, of a sudden irritation in the spread-out terminations of the optic nerve behind the retina. This would rapidly propagate itself along the nerve itself to the brain, where it would be distributed in various directions, becoming confused with other waves of irritation proceeding from independent centers, lingering here and there longer than elsewhere, and finally dying away.

In the recollection of a flash a similar sequence of events would take place, but they would occur in the reverse order. A variously distributed irritation in the brain, due to one or more of a multitude of possible causes, into which we need not stop to inquire, would propagate itself outward, becoming fainter the farther it traveled. The same links of the same nervous chain would be concerned in both cases, but the tension would be differently distributed among them. "When the faculty of sight-memory is strong, the vigorous propagation of a central impulse toward the optic nerve must be habitual; when it is weak, the propagation will not take place except in peculiar states