2. As to the Medium, I know not how that, by any contrivance of Art, can advantage Direct Hearing, otherwise than I have declar'd already in the propagation or conveyance of Sounds, though to the Refracting or Reflecting of them it may very much conduce; of which presently.
And so I have done with the first part of my present undertaking, which is the Comparison of Direct Vision and Audition, as to their Improvements from Art. The rest follow. Wherefore,
II. Concerning Refracted Vision and its Comparison, I observe, That Refracted Vision is always made Ex parte Medii, as Reflected is ex parte Objecti. And therefore, though Direct Vision may be help'd ex parte Objecti, Medii vel Organi, yet Refracted can be improv'd only ex parte Medii, and Reflected ex parte Corporis oppositi alone. Unless it be in a mixt or compound Vision, that is Refracto-Reflext, when the reflext Rays pass to the Eye through a refracting Medium, such as the Medium Internum, contain'd in the Body of the Eye, always is. So that in truth, all Vision is Refracted by an internal Refraction made in ipso Oculo.
And all that I have spoken of Vision holds true of Hearing also, both Refracted and Reflext, and therefore need not be repeated.
Refracted Vision arises from the different Density, Figure, and Magnitude of the Medium, which is somewhat alter'd also by the diverse incidence of the visible Rays. And so it is in Refracted Hearing, all these Causes concur to its Production, and some others to be hereafter consider'd.
Now as any Object (a Man for example) seen through a thicken'd Air, by Refraction appears greater than really he is: So likewise a Sound, heard through the same thicken'd part of the Atmosphere, will be considerably vary'd from what it would seem to be, if heard through a thinner Medium.
And this I call a Refracted Sound: But what this Refraction of Sound is, and how caus'd, may hereafter be discuss'd, when the Nature, and Motion, or Progression of Sounds are well stated.
For the Improvement of Refracted Vision artificial Instruments have been made, by grinding or blowing Glasses, into a certain Figure, and placing them at due distances, whereby the Object may be (as 'twere) enabled to send forth its Rays more vigorously, and the Visive Faculty impower'd the better to receive them. And thus also Instruments may be contriv'd for the assisting both the Sonorous Body, to send forth its Sound more strongly, and the Acoustick Faculty, to receive and discern it more easily and clearly. For,
1. As a fine Glass Bubble, fill'd with clear Water, and placed before a burning Candle or Lamp, does help it to dart forth its Rays to a prodigious Length and Brightness: So an Instrument may be invented, that apply'd to the Mouth (or any Sonorous Body) shall send forth the Voice distinctly to as prodigious a Distance and Loudness.
For if the Stentoro-phonecon (which is but a rude and unartificial Instrument) does such great feats, what might be done with one compos'd according to the Rules of Art? whose make should comply with the Laws of Sonorous Motion (which that does not) and therefore not so much Refract, as to alter and confound the Tone of the Voice and Words (as that somewhat does.)