Page:Philosophical Transactions - Volume 003.djvu/46

 thing, that may very well cause this loss of the Object. There are the Vessels of the Retina, the trunks whereof are big enough to given hindrance to Vision. These Vessels, which are no other but the ramifications of the Veins and Arteries, are derived from the Heart, and having no communication with the brain, they cannot carry thither the Species of the Objects, If therefore the Visual rays, issuing from an Object, fall on these Vessels at the place of their Trunk or main Body, 'tis certain that the Impression, made thereby, will produce no Vision, and that the picture of that Object will be deficient; as when on a white paper in an obscure Chamber, there is some black spot, or in it some hole considerably bigg: for the more sensible this blackness or hole is, the more of the image of the object it intercepts from our Eyes, It is not so in respect of the small ramifications, that issue from those trunks, and shoot into the Retina, For if they be met with at the place of the bottom of the Eye, where Vision is made distinct, they will not render the image of the Object deficient, because they are so small, as not to be sensible. Thus it is, that in Looking-glasses, when they want lead or tin in any place big enough to be perceived, the image, we there see, appears to have a hole, which happens not, when there is but so small a one, as might be made by the point of a needle.

Thus much being observed as to the deduction, made from this Experiment, I shall further note, That that paper, the sight whereof is lost, must be further or nearer off, according to the diversity of the structure of Eyes. For some loose this paper at the distance of two feet, some at a less, others at at greater distance; some loose it a little higher, others a little lower, according as the trunks of the vessels are situated in respect of the Optick Nerves and some loose more of it than others, according as those vessels are bigger or smaller. And because 'tis hard to determine precisely the place, where the object is lost in all sorts of Eyes we have ground to believe, that this loss is not always made on the extent of the Nerve, where the Retina is, but sometime besides it, where the Chloroides is found. For the trunks of the Vessels of the Retina are big and long enough to extend on this or that side of the Nerve, and so to hide some part of the Chloroides, according to their Magnitude, And in this case it will Rh