=The so-called 'Law of Relativity.'=--Weber's law seems only one case of the still wider law that the more we have to attend to the less capable we are of noticing any one detail. The law is obvious where the things differ in kind. How easily do we forget a bodily discomfort when conversation waxes hot; how little do we notice the noises in the room so long as our work absorbs us! _Ad plura intentus minus est ad singula sensus_, as the old proverb says. One might now add that the homogeneity of what we have to attend to does not alter the result; but that a mind with two strong sensations of the same sort already before it is incapacitated by their amount from noticing the detail of a difference between them which it would immediately be struck by, were the sensations themselves weaker and consequently endowed with less distracting power. This particular idea may be taken for what it is worth.[7] Meanwhile it is an undoubted general fact that the psychical effect of incoming currents does depend on what other currents may be simultaneously pouring in. Not only the _perceptibility_ of the object which the current brings before the mind, but the _quality_ of it, is changed by the other currents. "Simultaneous[8] sensations modify each other" is a brief expression for this law. "We feel all things in relation to each other" is Wundt's vaguer formula for this general 'law of relativity,' which in one shape or other has had vogue since Hobbes's time in psychology. Much mystery has been made of it, but although we are of course ignorant of the more intimate processes involved, there seems no ground to doubt that they are physiological, and come from the interference of one current with another. A current interfered with might naturally give rise to a modified sensation. Examples of the modification in question are easy to find.[9] Notes make each other sweeter in a chord, and so do colors when harmoniously combined. A certain amount of skin dipped in hot water gives the perception of a certain heat. More skin immersed makes the heat much more intense, although of course the water's heat is the same. Similarly there is a _chromatic minimum_ of size in objects. The image they cast on the retina must needs excite a sufficient number of fibres, or it will give no sensation of color at all. Weber observed that a thaler laid on the skin of the forehead feels heavier when cold than when warm. Urbantschitsch has found that all our sense-organs influence each other's sensations. The hue of patches of color so distant as not to be recognized was immediately, in his patients, perceived when a tuning-fork was sounded close to the ear. Letters too far off to be read could be read when the tuning-fork was heard, etc., etc. The most familiar examples of this sort of thing seem to be the increase of _pain_ by noise or light, and the increase of _nausea_ by all concomitant sensations.