different shades
of gray), yield all the color effects known to the human eye. Herschel
estimates that the workers on the mosaics at Rome must have
distinguished 30,000 different color tones. The _hue_ of a color refers
to its fundamental quality, as red or yellow; the _chroma_, to its
saturation, or the strength of the color; and the _tint,_ to the amount
of brightness (i.e., white) it contains.

HEARING.--Hearing is also a distance sense. The action of its stimulus
is mechanical, which is to say that the vibrations produced in the air
by the sounding body are finally transmitted by the mechanism of the
middle ear to the inner ear. Here the impulse is conveyed through the
liquid of the internal ear to the nerve endings as so many tiny blows,
which produce the nerve current carried to the brain by the auditory
nerve.

The sensation of hearing, like that of sight, gives us two qualities:
namely, _tones_ with their accompanying pitch and timbre, and _noises_.
Tones, or musical sounds, are produced by isochronous or equal-timed
vibrations; thus _C_ of the first octave is produced by 256 vibrations a
second, and if this tone is prolonged the vibration rate will continue
uniformly the same. Noises, on the other hand, are produced by
vibrations which have no uniformity of vibration rate. The ear's
sensibility to pitch extends over about seven octaves. The seven-octave
piano goes down to 27-1/2 vibrations and reaches up to 3,500 vibrations.
Notes of nearly 50,000 vibrations can be heard by an average ear,
however, though these are too painfully shrill to be musical. Taking
into account this upper limit, the range of the ear is about eleven
octaves. The ear, having given us _loudness_ of tones, which depends on
the amplitude of the vibrations, _pitch_, which depends on the rapidity
of the vibrations, and _timbre_, or _quality_, which depends on the
complexity of the vibrations, has no further qualities of sound to
reveal.

TASTE.--The sense of taste is located chiefly in the tongue, over the
surface of which are scattered many minute _taste-bulbs_. These can be
seen as small red specks, most plentifully distributed along the edges
and at the tip of the tongue. The substance tasted must be in
_solution_, and come in contact with the nerve endings. The action of
the stimulus is _chemical_.

The sense of taste recognizes the four qualities of _sour_, _sweet_,
_salt_, and _bitter_. Many of the qualities which we improperly cal