Empedocles says that these images are caused by certain effluvias which, meeting together and insisting upon the superficies of the mirror, are perfected by that fiery quality emitted by the said mirror, which transmutes withal the air that surrounds it. Democritus and Epicurus, that the specular appearances are formed by the subsistence of the images which flow from our eyes; these fall upon the mirror and remain, while the light rebounds to the eye. The followers of Pythagoras explain it by the reflection of the sight; for our sight being extended (as it were) to the brass, and meeting with the smooth dense surface thereof it is struck back, and caused to return upon itself: the same appears in the hand, when it is stretched out and then brought back again to the shoulder. Any one may apply these instances to explain the manner of seeing. CHAPTER XV. WHETHER DARKNESS CAN BE VISIBLE TO US. The Stoics say that darkness is seen by us, for out of our eyes there issues out some light into it; and our eyes do not impose upon us, for they really perceive there is darkness. Chrysippus says that we see darkness by the striking of the intermediate air; for the visual spirits which proceed from the principal part of the soul and reach to the ball of the eye pierce this air, which, after they have made those strokes upon it, presses conically on the surrounding air, where this is homogeneous. For from the eyes those rays are poured forth which are neither black nor cloudy. Upon this account darkness is visible to us CHAPTER XVI. OF HEARING. Empedocles says that hearing is formed by the insidency of the air upon the spiral, which it is said hangs within the ear as a bell, and is beat upon by the air. Alcmaeon, that the vacuity that is within the ear makes us to have the sense of hearing, for the air forcing a vacuum gives the sound; every inanity affords a ringing. Diogenes, the air which is in the head, being struck upon by the voice, gives the hearing. Plato and his followers, the air which exists in the head being struck upon, is reflected to the principal part of the soul, and this causeth the sense of hearing. CHAPTER XVII. OF SMELLING. Alcmaeon believes that the principal part of the soul, residing in the brain, draws to itself odors by respiration. Empedocles, that scents insert themselves into the breathing of the lungs; for, when there is a great difficulty in breathing, odors are not perceived by reason of the sharpness; and this we experience in those who have the defluxion of rheum. CHAPTER XVIII. OF TASTE. Alcmaeon says that a moist warmth in the tongue, joined with the softness of it, gives the difference of taste. Diogenes, that by the softness and sponginess of the tongue, and because the veins of the body are joined in it, tastes are diffused by the tongue; for they are attracted from it to that sense and to the commanding part of the soul, as from a sponge. CHAPTER XIX. OF THE VOICE.