(*70 
be true, that Vifion is not made in all the parts where the Ch~ 
roides is found, though they be expofed to the light. Which 
may very well give a check to your opinion, forafmuch as thofe 
trunks would hinder the obje<fis,falling on them, from coming to 
the Choroides $ which would render the image deficient in that 
place, in regard that thofe [pedes would not be able to make an 
imprelTion on the Organ of Vifion through thofe veffels. 
In the mean time, fo pretty a difcovery, as this is, could not 
belong without being confirmed. For as the fecret of your Ex- 
periment confifts in contrivingpthat the pidtureofanObjed may 
fall juft on the Optick Nerve, or thereabout, M. Vicar d hath de* 
vifed a way, by which an objedi is loft keeping both Eyes opeo> 
by letting the image of the objedt fall on both the Optick Nerves 
at one and the fame time, after this manner 3 
Faften againft a wall a round white paper, of the bignefs of an 
inch or two, and on the fide of this paper put two marks one 
on the right, the other on the left fide, each about two foot di- 
ftant 5 then place your felf dire&ly before the Paper, at thedi- 
fiance of about nine foot, and put ^ha End of your finger over 
againft your both Eyes , fo that it may hide from the right 
Eye the left mark, and from the left Eye, the right 
mark. If you remain firm in that pofture, and look fteddily with 
both Eyes on the end of your finger, the paper, which is not at all 
cover'd thereby, will altogether difappear 5 which muft be the 
morefurprifing, becaufe that without this particular encounter 
of the Optick Nerves, where no Vifion is made, the paper would 
appear double^ as you willfinde as often as the finger fliall not be 
placed as it ought to be, or when the fight is carried any thing 
fideways whereof the reafon is fufficiently known. 
The application of this way is eafily made to that of yours. 
For when one looks fteddily with both Eyes on the End of one's 
finger, held before the marks, 'tis the fame thing, as if you dire- 
6ted each eye by it felf to the place, which is to be looked on to 
loofe the paper 5 So that one may with both Eyes do the fame 
thing, that you do mth m^ keeping the other clofed,&c r 
