C 266 ) 
hard and thick ; nor have a (limy or dirdnefs upon it> to 
hinder the Imprcffion of light* nor want a Communication 
with the Cptich^ Nerve^ 
3. If the want of Vi^on in the foregoing Experiment, 
may be falved by any of the Two probable reafons here 
offered ^ then there is no need of difcharging the l{etina. 
To the Firft of thefe Mr. Mariotte anfwers s That there 
are defcds in f^fion cznikd by the 5W f^ejfeb in the ^tt* 
na Cand he proves it by a remarkable as well as new Expe- 
riment) bqt thefe defects arc not fenfible when we look 
with both Eyes 5 for' there sire no f^effels that lye foneac 
the ^A;^^?/^^r^Af as to hinder a direct view^ and in an Ob- 
lique, one Eye helps the other : it being difficult that the 
J{ays (houldfall on a like Plane in each Eye. Again thefe 
Feffels that are neareft the uixis Opticus^ are no bigger than 
a//^?r , or the 740^^ part of an Inch; and being in the 
furfaceofthe7^^^/w^> areatfbme diftance from th^ cho^ 
roide , fo as to let J{ays enough pafs under , for the di- 
ftinguifliing ofoZy^Sj- net very fmall. T\it Vefiels s\(6 that 
cirry the blood are clear and PeUucid^ cm(\ng a l{efraHion 
that is helpful to r^^ow. 
Here alfo may enter fome general confiderations as that 
thcimpreffion of a luminous Oiy>^ remains fometime in 
the Or^an : that fome Fibres being ftrongly moved^ others 
near them are alfo in motion : that the Eyes are always in 
motion, and very hard to be fixt in one place, thoit 
were defired. 
To the Second head he anfwers^ That the concavity 
ofthef/^^ra^ cannot be very rugged i for upon the dif- 
fering an Eye, and removeing the Retina , the furface of 
the Choroide has refle<n:ed an Objed: as diftin(5Hy as a con- 
cave Speculum, That there appears no fbyls or dirtinefs > 
till the outward Cidicle be broke, and then the Organ is 
difoiderd. As for the ihicknefs of it* he fays he finds 
it in a man but as a flieet of Paper * or the Via Ma^ 
ter in the brain. That the Blood Vejfels are weaved to- 
gether 
