been observed already. But the nearness and remoteness of the luminous object are the respective associates of these. The muscular rings therefore, which at first contracted only when very luminous objects approached, will afterwards contract when moderately luminous ones do. And thus vision will be made distinct in general. But distinct vision, by recurring, will perpetuate and perfect itself, and indistinct check and abolish itself, from the agreeableness and disagreeableness accompanying them respectively, by Prop. XXII. Cor. I. till at last the child gets a perfectly voluntary power of suiting his eyes to the distance. Adults seem to have a power of preparing the eyes previously to see at a proposed distance; and some to have a semi-voluntary power of contracting and dilating the pupil, viz. by fancying a bright object near, and a dark one far off; though the quantity of light which falls upon the eye remains the same.
Section V
THE SENSE OF HEARING.
The immediate organ of hearing appears to be the soft portion of the seventh pair of nerves, distributed in the cochlea, and semi-circular canals. What the particular uses of these cavities are, is not known. They bear some obscure likeness to the instruments commonly made use of for increasing either the loudness of sounds, or the effects of them upon the ear; just as the coats and humours of the eye resemble lenses. The auditory nerve is also like the optic in detaching no branches off to the neighbouring parts; and there are many other instances of resemblance between these two most refined and spiritual, if one may so say, of our senses; some of which I shall mention in the course of this Section.
The auricle and meatus auditorius are cartilaginous, and seem by this means to be peculiarly fitted for receiving and retaining the vibrations of the air, and for communicating them to the membrana tympani.
This membrane appears to be kept in a state of constant tension by muscles, that act upon the small bones in different ways, and thus to be fitted for vibrating synchronously to the several sounds which affect it. Agreeably to which, the degree of tension varies, so as to be less when the musculus externus and
