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_. 1. As a _Plain Speculum_ reflects the _Object_ in its due _Dimensions_ and _Colours_; allowing for their difference of appearance, according to their distance: So a _Plane Corpus Obstans_ reflects the _Sound_ back in its due _Tone_ and _Loudness_; if allowance be likewise made for the proportionable decrease of the Sound, according to its distance. 2. As a _Convex Speculum_ reflects the _Object less_, but somewhat _brighter_ or clearer: So a _Convex Corpus Obstans_ repels the Sound (insensibly) _smaller_; but somewhat _quicker_ (though _weaker_) than otherwise it would be.