In like manner a stimulus greater than natural, applied to a part of the
system, increases the exertion of sensorial power in that part, and
diminishes it in some other part. As in the commencement of scarlet fever,
it is usual to see great redness and heat on the faces and breasts of
children, while at the same time their feet are colder than natural;
partial heats are observable in other fevers with debility, and are
generally attended with torpor or quiescence of some other part of the
system. But these partial exertions of sensorial power are sometimes
attended with increased partial exertions in other parts of the system,
which sympathize with them, as the flushing of the face after a full meal.
Both these therefore are to be ascribed to sympathetic associations,
explained in Sect. XXXV. and not to general exhaustion or accumulation of
sensorial power.

2. A quantity of stimulus greater than natural, producing an increased
exertion of sensorial power in any particular organ, diminishes the
quantity of it in that organ. This appears from the contractions of animal
fibres being not so easily excited by a less stimulus after the organ has
been subjected to a greater. Thus after looking at any luminous object of a
small size, as at the setting sun, for a short time, so as not much to
fatigue the eye, this part of the retina becomes less sensible to smaller
quantities of light; hence when the eyes are turned on other less luminous
parts of the sky, a dark spot is seen resembling the shape of the sun, or
other luminous object which we last behold. See Sect. XL. No. 2.

Thus we are some time before we can distinguish objects in an obscure room
after coming from bright day-light, though the iris presently contracts
itself. We are not able to hear weak sounds after loud ones. And the
stomachs of those who have been much habituated to the stronger stimulus of
fermented or spirituous liquors, are not excited into due action by weaker