By the help hereof we may hear the different Cries and Tones, as well as by Microscopes see the divers Shapes and Figures of the smallest Animals.
4. As by Polyscopes or Multiplying-Glasses, one thing is represented to the Eye as many, whether in the same or different Shapes (for so Multiplying-Glasses may be contriv'd:) So by a Polyphone or Polyacoustick well order'd, one Sound may be heard as many, either of the same or a different Note. Insomuch, that who uses this Instrument, he shall, at the Sound of a single Viol, seem to hear a whole Consort and all true Harmony. By which means this Instrument has much the advantage of the Polyscope.
And thus much may suffice for comparing the Improvements made upon Refracted Seeing and Hearing; I call it Refracted Hearing, because made through a Medium, viz. thick Air, or an Instrument, through which the Sound passing is broken or refracted.
III. Reflected Vision has been improv'd by the Invention of Looking-glasses and Polished Metals, whether Plane, Concave, or Convex; and these two last, either Spherical, Oval, Cylindrical, Conical, Hyperbolical, or of several other shapes; all which cause a different Reflection, and vary the Phænomena.
Thus also Reflext Audition, made by Ecchoes, may be improv'd, by contriving several sorts of Artificial Ecchoes; as 'tis no hard matter to do in almost any place.
For (speaking in the general) Any Sound, falling directly or obliquely upon any dense Body, of a smooth (whether Plane or Arch'd) Superficies, is beat back again and reflected, or does eccho more or less.
I say (1.) falling directly or obliquely; because, if the Sound be sent out and propagated parallel to the Surface of the Dense Body, or be made so far off and so weak, that it cannot reach it, there will be no Reflection of Sound, no Eccho.
I say (2.) upon a Body of a smooth Superficies; because if the Surface of the Corpus Obstans be uneven, the Air by reverberation will be put out of its regular Motion, and the Sound thereby broken and extinguish'd: So that tho' in this case also the Air be beaten back again, yet Sound is not reflected, nor is there any Eccho.
I say (3.) it does eccho more or less, to shew, that when all things are, as is before describ'd, there is still an Ecchoing, though it be not always heard; either because the direct Sound is too weak to be beaten quite back again to him that made it; or that it does return home to him, but so weak, that without the help of a good Otacoustick, it cannot be discern'd; or that he stands in a wrong place, to receive the reflected Sound, which passes over his head, under his Feet, or to one side of him; which therefore may be heard by a Man standing in that place, where the reflected Sound will come, provided no interpos'd Body does intercept it; but not by him, that first made it.
I shall further make out the comparison 'twixt Reflex'd Vision and Audition, by these following Propositions.