Page:Mathematical collections and translations, in two tomes - Salusbury (1661).djvu/86

 See here is the glasse, which I would have to be placed close to the other. But first let us go yonder towards the reflection of that flat one, and attentively observe its clarity; see how bright it is here where it shines, and how distinctly one may discern these small unevennesses in the Wall.

I have seen and very well observed the same, now place the other glasse by the side of the first.

See where it is. It was placed there assoon as you began to look upon the Walls small unevennesses, and you perceived it not, so great was the encrease of the light all over the rest of the Wall. Now take away the flat glasse. Behold now all reflection removed, though the great convex glasse still remaineth. Remove this also, and place it there again if you please, and you shall see no alteration of light in all the Wall. See here then demonstrated to sense, that the reflection of the Sun, made upon a spherical convex glasse, doth not sensibly illuminate the places neer unto it. Now what say you to this experiment?

I am afraid that there may be some Leigerdemain, used in this affair; yet in beholding that glasse I see it dart forth a great splendor, which dazleth my eyes; and that which imports most of all, I see it from what place soever I look upon it; and I see it go changing situation upon the superficies of the glasse, which way soever I place my self to look upon it; a necessary argument, that the light is livelily reflected towards every side, and consequently, as strongly upon all that Wall, as upon my eye.

Now you see how cautiously and reservedly you ought to proceed in lending your assent to that, which discourse alone representeth to you. There is no doubt but that this which you say, carrieth with it probability enough, yet you may see, how sensible experience proves the contrary.

How then doth this come to pass?

I will deliver you my thoughts thereof, but I cannot tell how you may be pleas'd therewith. And first, that lively splendor which you see upon the glass, and which you think occupieth a good part thereof, is nothing near so great, nay is very exceeding small; but its liveliness occasioneth in your eye, (by means of the reflection made on the humidity of the extream parts of the eye-brows, which distendeth upon the pupil) an adventitious irradiation, like to that blaze which we think we see about the flame of a candle placed at some distance; or if you will, you may resemble it to the adventitious splendor of a star; for if you should compare the small body v. g. of the Canicula, seen in the day time with the Telescope, when it is seen without such irradiation, with the same seen by night by the eye it self, you will doubtless comprehend that being irradiated, it appeareth above a thousand