An Account of T^vpo Letters of Mr. Pe- 
rault, and Mr. Mariotte, concerning 
Vifion ^ Trinted at? arts i6Sz^ 
T^He Occafion of thefe Two letters^ was an Obfervation 
' of Mr. }Aar$ottes > that any ObjeU is not feen when the 
Species light upon the Baps of the Optic Nerve, The Ex- 
periment upon which it is grounded is this : take a piece 
ofwhite paper of Six Inches i>i^w^^^r, and faften ic upon 
a dark coloured Wall, that it may be level with your 
Eye; take another fmall piece of Paper and place it towards 
your Left hand , at Two Foot diftance from the former , 
but about Two Jnches higher on the Wall: if youthen 
remove to the diftance of Eight or Nine feet^ andclofe 
the left Eye, fixing the Right upon thefmaller piece of pa- 
per, the Larger paper will quite difappear. 
It is not at all doubted but t\it lma^e which Ihould appear 
falls juft upon the of the Optic Nerve ^ ic is alfo cer- 
tain that the l^etina h to be found in that place 3 but the 
Choroid mis which gives a very fair fufpition to Mr. M<«- 
riotte^ that the Umotde is the feat oiVifion^^Sii not the Ke- 
tina. 
Tht Novelty of this Opifjion hath found many Oppofers, 
and ^mong che reft Monjieur Permit^ whofe Arguments in 
the firff Letters are in fhort reduced to Three Heads* 
r. If the choreide were the feat of Vifion , its function 
would be hindred by the branches of Blood VeJJels lying in 
the jRjttna. 
a. The ^/^'(^w^^fhottld not be rugged and unequal ; nor 
hard 
