Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/356

 spects ought to be made more and lesse remote from the eye.

I see already, whither the apprehension of Sagredus, a most diligent observer of Natures secrets, tendeth; and I would lay any wager, that amongst the thousands that have observed Cats to contract and inlarge the pupils of their eyes very much, there are not two, nor haply one that hath observed the like effect to be wrought by the pupils of men in seeing, whilst the medium is much or little illumin'd, and that in the open light the circlet of the pupil diminisheth considerably: so that in looking upon the face or Discus of the Sun, it is reduced to a smallnesse lesser than a grain of * Panick, and in beholding objects that do not shine, and are in a lesse luminous medium, it is inlarged to the bignesse of a Lintel or more; and in summe this expansion and contraction differeth in more than decuple proportion: From whence it is manifest, that when the pupil is much dilated, it is necessary that the angle of the rayes concourse be more remote from the eye; which happeneth in beholding objects little luminated. This is a Doctrine which Sagredus hath, just now, given me the hint of, whereby, if we were to make a very exact observation, and of great consequence, we are advertized to make the observation of that concourse in the act of the same, or just such another operation; but in this our case, wherein we are to shew the errour of Astronomers, this accuratenesse is not necessary: for though we should, in favour of the contrary party, suppose the said concourse to be made upon the pupil it self, it would import little, their mistake being so great. I am not certain, Sagredus, that this would have been your objection.

It is the very same, and I am glad that it was not altogether without reason, as your concurrence in the same assureth me; but yet upon this occasion I would willingly hear what way may be taken to finde out the distance of the concourse of the visual rayes.

The method is very easie, and this it is, I take two long * labels of paper, one black, and the other white, and make the black half as broad as the white; then I stick up the white against a wall, and far from that I place the other upon a stick, or other support, at a distance of fifteen or twenty yards, and receding from this, second another such a space in the same right line, it is very manifest, that at the said distance the right lines will concur, that departing from the termes of the breadth of the white piece, shall passe close by the edges of the other label placed in the mid-way; whence it followeth, that in case the eye were placed in the point of the said concourse or intersection, the black slip of paper in the midst would precisely hide the op-