The eye, whose general structure is sufficiently described in all standard physiologies, consists of a visual apparatus designed to bring the images of objects to a clear focus on the retina at the _fovea_, or area of clearest vision, near the point of entrance of the optic nerve. The sensation of sight coming from this retinal image unaided by other sensations gives us but two qualities, _light and color_. The eye can distinguish many different grades of light from purest white on through the various grays to densest black. The range is greater still in color. We speak of the seven colors of the spectrum, violet, indigo, blue, green, yellow, orange, and red. But this is not a very serviceable classification, since the average eye can distinguish about 35,000 color effects. It is also somewhat bewildering to find that all these colors seem to be produced from the four fundamental hues, red, green, yellow, and blue, plus the various tints. These four, combined in varying proportions and with different degrees of light (i.e., 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.