Sound.—Its keenness; the appreciation of different grades of loudness; that of different notes.
Touch.—Discrimination of different roughnesses, such as wire-work of differently sized mesh.
Muscular Sense.—Discrimination of weights externally alike, but differing slightly in specific gravity.
Another class of delicate apparatus refers to the rate of response to stimuli. A signal is given to one of the senses, as by the sight of a suddenly lifted finger, by an exclamation, or by a touch, to which response is made by pressing a stop. The interval between the signal and the response is measurable, and it differs in different persons.
Another well-known arrangement tests the time lost in forming a simple judgment. Arrangement is made for two possible and different signals, which are severally to be responded to by different forms of response. The subject of the experiment is ignorant which of the two signals will appear. After he perceives it, there is an appreciable time of hesitation before he is able to make the appropriate response, and this time is easily measured, and is found to differ in different persons.
The persistence of impressions, especially if visual ones, is exceeding various. Some persons are strongly affected by after-images and others are not. For example, after gazing at a red wafer for a short definite time and then rapidly withdrawing the eye, the appearance of a green after-image will be present to some and not to others. There can be little doubt that the liability to after-images is an important factor of the artistic temperament, being the base of the enhanced susceptibility to conditions of contrast and harmony of colors. Numerous experiments exist bearing on various kinds of after-images, but they want systematizing for anthropometric purposes.
The memory, in its dependence on the relative impressions of eye, ear, and other senses, whether severally or in combination, admits of being tested, and here again numerous scattered experiences have been gained, and ingenious experiments have been devised which require consolidating and systematizing.
This is perhaps as much as need be said in a very brief general glance over a large division of a large subject. My object is to point out that means already exist for the appraisement of many of the principal bodily faculties, but that they require to be systematized, and that others have to be contrived, and that they can not be properly utilized for ordinary anthropometric purposes without such apparatus as would require to be kept in a laboratory and used under the guidance of an intelligent operator.
I will say a few words, and a few only, upon another large branch to which I alluded in my previous article, namely the medical life-history of each individual. There seems to be need for medico-metric