Distances of Objects also, ’tis very likely they distinguish, partly by the consonant impressions made in some two convenient Pearls, one in each cluster; for, according as those congruous impressions affect, two Pearls neerer approach’d to each other, the neerer is the Object, and the farther they are distant, the more distant is the Object: partly also by the alteration of each Pearl, requisite to make the Sensation or Picture perfect; for ’tis impossible that the Pictures of two Objects, variously distant, can be perfectly painted, or made on the same _Retina_ or bottom of the eye not altered, as will be very evident to any one that shall attentively consider the nature of refraction. Now, whether this alteration may be in the Figure of the _Cornea_, in the motion of access or recess of the _Retina_ towards the _Cornea_, or in the alteration of a crystalline humour, if such there be, I pretend not to determine; though I think we need not doubt, but that there may be as much curiosity of contrivance and structure in every one of these Pearls, as in the eye of a Whale or Elephant, and the almighty’s _Fiat_ could as easily cause the existence of the one as the other; and as one day and a thousand years are the same with him, so may one eye and ten thousand. This we may be sure of, that the filaments or sensative parts of the _Retina_ must be most exceedingly curious and minute, since the whole Picture it self is such; what must needs the component parts be of that _Retina_, which distinguishes the part of an object’s Picture that must be many millions of millions less then that in a man’s eye? And how exceeding curious and subtile must the component parts of the _medium_ that conveys light be, when we find the instrument made for its reception or refraction to be so exceedingly small? we may, I think, from this speculation be sufficiently discouraged from hoping to discover by any optick or other instrument the determinate bulk of the parts of the _medium_ that conveys the pulse of light, since we find that there is not less accurateness shewn in the Figure and polish of those exceedingly minute lenticular surfaces, then in those more large and conspicuous surfaces of our own eyes. And yet can I not doubt, but that there is a determinate bulk of those parts, since I find them unable to enter between the parts of Mercury, which being in motion, must necessarily have pores, as I shall elsewhere shew, and here pass by, as being a digression. As concerning the horns FF, the feelers or smellers, GG, the _Proboscis_ HH, and I, the hairs and brisles, KK, I shall indeavour to describe in the 42. _Observation_. * * * * * Observ. XL. _Of the Teeth of a _Snail_._