Weight can be measured with great accuracy by the balance, but the balance is an invention like the clock. It was the need of weighing which led to the invention of the balance, not the balance which led to the discovery of weight. Apart from our sensations of varying intensity, physical science can only define weight in terms of size and density, density in terms of force, and force in terms of time and space.
16. There is a threshold of intensity.
When stimuli are very weak, they are accompanied by no distinct sensation. This may be partly a physiological fact, due to inertia of the sense-organ and nerve. But such stimuli may in certain cases be perceived by an effort and in other cases affect the course of mental life, although no amount of effort can call them to distinct consciousness. The intensity of a stimulus which can just be perceived (or perceived in a given percentage of cases) may be measured. This minimum perceptible is a function of the time of stimulation and of the degree of attention. Thus it has been found that the time a color must work on the retina in order that it may be just recognized, tends to vary as the logarithm of the intensity of the light. If we learn to measure the effort of attention, it may in like manner be correlated with the intensity of the stimulus which can just be perceived. It is, also, possible to compare the effects on mental life of stimuli too faint to be perceived with the effects of such as are distinctly perceived, and thus to correlate the intensity of sub-consciousness with that of consciousness.
17. The error of observation is a function of the intensity of sensation.
Stimuli may be so nearly alike in intensity that no difference can be detected by common observation. By proper methods the error of observation may be determined, and this is an important anthropometric test. It measures class and individual
