Vision is performed by having a picture, formed by the rays of light which are reflected from the object, painted in one piece, instantaneously, on the retina, or last nervous part of the eye.  Or, according to others, there is but one point of any object painted on the eye in such a manner as to be perceived at once, but by moving the eye, we gather up, with great celerity, the several parts of the object, so as to form one uniform piece.  If the former opinion be allowed, it will be considered,[35] that though all the light reflected from a large body should strike the eye in one instant; yet we must suppose that the body itself is formed of a vast number of distinct points, every one of which, or the ray from every one, makes an impression on the retina.  So that, though the image of one point should cause but a small tension of this membrane, another, and another, and another stroke, must in their progress cause a very great one, until it arrives at last to the highest degree; and the whole capacity of the eye, vibrating in all its parts, must approach near to the nature of what causes pain, and consequently must produce an idea of the sublime.  Again, if we take it, that one point only of an object is distinguishable at once; the matter will amount nearly to the same thing, or rather it will make the origin of the sublime from greatness of dimension yet clearer.  For if but one point is observed at once, the eye must traverse the vast space of such bodies with great quickness, and consequently the fine nerves and muscles destined to the motion of that part must be very much strained; and their great sensibility must make them highly affected by this straining.  Besides, it signifies just nothing to the effect produced, whether a body has its parts connected and makes its impression at once; or, making but one impression of a point at a time, it causes a succession of the same or others so quickly as to make them seem united; as is evident from the common effect of whirling about a lighted torch or piece of wood:  which, if done with celerity, seems a circle of fire.

SECTION X.

UNITY WHY REQUISITE TO VASTNESS.

It may be objected to this theory, that the eye generally receives an equal number of rays at all times, and that therefore a great object cannot affect it by the number of rays, more than that variety of objects which the eye must always discern whilst it remains open.  But to this I answer, that admitting an equal number of rays, or an equal quantity of luminous particles to strike the eye at all times, yet if these rays frequently vary their nature, now to blue, now to red, and so on, or their manner of termination, as to a number of petty squares, triangles, or the like, at every change, whether of color or shape, the organ has a sort of relaxation or rest; but this relaxation and labor so often interrupted, is by no means productive of ease; neither has it the effect of vigorous and