will leave off to make the Hours, if the Movements of it be broken. "In fine, these Primary and indivisible Atomes make a Circle, whereon without difficulty move the most perplexed Difficulties of Natural Philosophy; not so much as even the very Operation of the Senses, which no Body hitherto hath been able to conceive, but I will easily explain by these little Bodies. Let us begin with the Sight. It deserves, as being the most incomprehensible, our first Essay. [Sidenote: Operation of the Senses] [5]"It is performed then, as I imagine, when the Tunicles of the Eye, whose Pores resemble those of Glass, transmitting that fiery Dust which is called Visual Rays, the same is stopt by some opacous Matter which makes it recoil; and then, meeting in its retreat the Image of the Object that forced it back, and that Image being but an infinite number of little Bodies exhaled in an equal Superfice from the Object beheld, it pursues it to our Eye: You'll not fail to Object, I know, that Glass is an Opacous Body, and very Compact; and that nevertheless, instead of reflecting other Bodies, it lets them pass through: But I answer, that the Pores of Glass are shaped in the same Figure as those Atomes are which pass through it; and as a Wheat-Sieve is not proper for Sifting of Oats, nor an Oat-Sieve to Sift Wheat; so a Box of Deal-Board, though it be thin and lets a sound go through it, is impenetrable to the Sight; and a piece of Chrystal, though transparent and pervious to the Eye, is not penetrable to the Touch." I could not here forbear to interrupt him: "A great Poet and Philosopher[6] of our World," said I, "hath after _Epicurus_ and _Democritus_[7] spoken of these little Bodies, in the same manner almost as you do; and therefore, you don't at all surprise me by that Discourse: Only tell me, I pray, as you proceed, how, according to your Principles, you'll explain to me the manner of drawing your Picture in a Looking-Glass." "That's very easie," replied he, "for imagine with your self, that those Fires of our Eyes, having passed through the Glass and meeting behind it an Opacous Body that reverberates them, they come back the way they went; and finding those little Bodies marching in equal Superfices upon the Glass, they repel them to our Eyes; and our Imagination, hotter than the other Faculties of our Soul, attracts the more subtile, wherewith it draws our Picture in little.