Page:Philosophical Transactions - Volume 003.djvu/47

 be true, that Vision is not made in all the parts where the Choroides is found, though they be exposed to the light. Which may very well give a check to your opinion, forasmuch as those trunks would hinder the objects, falling on them, from coming to the Choroides which would render the image deficient in that place, in regard that those species would not be able to make an impression on the Organ of Vision through those vessels.

In the mean time, so pretty a discovery, as this is, could not belong without being confirmed. For as the secret of your Experiment consists in contriving, that the picture of an Object may fall just on the Optick Nerve, or thereabout, M. Picard hath devised a way, by which an object is lost keeping both Eyes open, by letting the image of the object fall on both the Optick Nerves at one and the same time, after this manner;

Fasten against a wall a round white paper, of the bigness of an inch or two, and on the side of this paper put two marks one on the right, the other on the left side, each about two foot distant, then place your self directly before the Paper, at the distance of about nine foot, and put the End of your finger over against your both Eyes, so 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 posture, and look steddily with both Eyes on the end of your finger, the paper, which is not at all cover'd thereby, will altogether disappear; which must be the more surprising, because that without this particular encounter of the Optick Nerves, where no Vision is made, the paper would appear double, as you will finde as often as the finger shall not be placed as it ought to be, or when the sight is carried any thing sideways; whereof the reason is sufficiently known.

The application of this way is easily made to that of yours. For when one looks steddily with both Eyes on the End of one's finger, held before the marks, 'tis the same thing, as if you directed each eye by it self to the place, which is to be looked on to loose the paper; So that one may with both Eyes do the same thing, that you do with one, keeping the other closed, &c.

Rh