differences, and demonstrates the results of education, occupation, etc. The error of observation is found in most cases to become greater as the intensity of the sensation becomes greater. Thus we can distinguish (by lifting) 6 ounces from 5, but not 26 ounces from 25. When we determine the curve and equation representing the relation between the error of observation and the intensity of the stimulus, we define mental relations in the units of physical science. We can, again, determine the relation of the error of observation to the time of stimulation and to the time of perception, and the correlation of the time and intensity of sensation. Through the error of observation the rate at which a simple sensation fades from the memory may be measured. If we could do this one thing only, the possibility of a mental mechanics would be demonstrated. It is thought by Fechner and others that we can measure the intensity of sensation by means of the difference which can just be noticed. If this difference, as some experiments seem to show, is proportional to the intensity of the stimulus, and if it may be regarded as a unit for measuring sensation, it follows that the intensity of a sensation varies as the logarithm of the intensity of the stimulus. It is, further, possible that the intensity of a sensation, and more especially of an emotion, may be measured by the resulting or accompanying bodily movement.
18. The intensity of pleasures and pains may, perhaps, be measured.
In this case the just noticeable difference is a more likely unit than in the case of sensations, for even though the just noticeable change may not be an equal change at different degrees of intensity, it would seem to be an equally desirable change. Some approximation to the logarithmic relations seems, indeed, to hold as regards the intensity of pleasure, and the wealth which purchases certain pleasures. With an income of $1000, $1 will go about as far as $10 with an income of $10,0000. A man in public life would scarcely be affected by
