64 THE POPULAR SCIENCE MONTHLY. 



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, comj^letes an 

 electrical equipment that leaves little to be desired in the way of se- 

 curity and convenience. 



MENTAL IMAGEKY. 



By FRANCIS GALTON, F. E. S. 



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 memoi'ies 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 aj)pearance 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 contrast- 

 ing 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 irri- 

 tation in the sj^read-out terminations of the oj^tic nerve behind the 

 retina. This would raj^idly propagate itself along the nerve itself to 

 the brain, where it would be distributed in various directions, becom- 

 ing confused with other waves of irritation proceeding from indepen- 

 dent 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 prop- 

 agate 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 thein. 

 "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 



