. C48bl 
conrairi'd in the bofdy of the Eye, always is. So that in 
truth all Vifion is ^fr ailed ^ by an Internal Refraction 
ma,de in ip/o Oculo. 
And all that I have fpoken of Vi/ion holds true of Hear- 
ing alfo, both I{fraSed^nd l{eflext, and therefore need 
not be repeated. 
l^fraBed Vijion arifes from the different Dtnjity , Fi^ 
gure'xwd Magnitudeoi the Medium ; which is foraewhat 
alterd alfoby the divers Incidence of the Vifible raies. And 
fo it is in B^fr ailed Hearing , all thefe Caufes concur 
to its produdion, and fome others to be hereafter con- 
fider'd. 
Now as any Objed: ( a man for example ) feen through 
a thicken d air, by i(^/r<3^S?^;2 appears greater then really 
he is : So likewife a Sounds heard through the lame thick- 
ned part of t\vQ Jtmofphear, will be confiderably vary'd 
from what it would feem to be, if heard through a- 
thinner Medium. 
And this I call ^'E^fra'tled Sound. But what this Re- 
fradrion of Sound is, and how caus'd, paay hereafter be 
difcufs'd, when the nature and motion or progreflion of 
Sounds are well ftated. 
For the Improvement of l{efraUed Vijion artificial In- 
ftruments have been made, by grinding or blowing Glaf- 
fes into a certain Figure, and placing them at due di- 
itances j whereby the Objed: may be as 'twere ) enabled 
to fend forth it s raies more vigoroufly , and the Vifive 
Faculty impowered the better to receive them And thus 
aifoinftruments may be contrived for the affiftmg both 
xki^ Sonorous Body, to fend forth its Sound more ftrongly, 
and the Acoujlic\Faculty to receive and difcern it more 
eafily and clearly. 
For , 
I. As afine Glajs 5//^^/^, fiU'd with clear water, and 
plac'd before a burning Candle or Lamp, does help it to 
dart forth it's raies to a prodigious length and brightnefs:: 
So 1 
