Concerning the comparison, your _Author_ makes between an Immaterial Spirit, and Light,[1] That, _as Light is contractive and dilative, and yet not divisible, so is also an Immaterial substance._ Give me leave to tell you, that in my opinion, all that is contractive and dilative, is also dividable, and so is light: As for example; when a Candle is snuff'd, the Snuffers do not onely clip the wick, but also the light: The like when a dark body is interposed, or crosses the rays of the Sun; it cuts those rays asunder, which by reason they cannot joyn together again, because of the interposed body, the light cut off, suddenly goeth out; that is, the matter of light is altered from the figure of light, to some other thing, but not annihilated: And since no more light can flow into the room from the Fountain or Spring of Light, the Sun, because the passage is stopt close, the room remaineth dark: For Light is somewhat of the nature of Water; so long as the Spring is open, the Water flows, and whatsoever is taken away, the Spring supplies; and if another body onely presses thorow it, it immediately joyns and closes its severed parts again, without any difficulty or loss; The same doth Light; onely the difference is, that the substance of Light is extraordinary rare, and pure; for as Air is so much rarer then Water, so Light is so much rarer and purer then Air, and its matter may be of so dilating a nature, as to dilate from a point into numerous rayes. As for ordinary Fire-light, it doth not last longer, then it hath fuel to feed it, and so likewise it is with the light of the Sun; for Light is according to the substance that feeds it; and though it is a substance it self, yet it increases and decreases, according as it hath something that succours or nourishes it. But some may object, that if Light were a body, and did contract and dilate, as I say, it is impossible that it could display it self in so great and vast a compass, and remove so suddenly and instantly as it doth. To which objection, I answer, first, That although I say, Light is a real corporeal substance, and doth contract and dilate it self from a point into numerous rayes, as also in another Letter I sent you before,[2] That Light and Darkness do succeed each other; nevertheless, as for the perception of Light, I am not so eager in maintaining this opinion, as if it was an Infallible Truth, and impossible to be otherwise; but I say onely, That, to my sense and reason, it seems very probable, that it may be so, that the light of the Sun doth really dilate it self into so vast a compass as we see, and that light and darkness do really succeed each other, as all other Creatures do: But yet it seems also probable to mee, that the parts of the Air may onely pattern out the figure of light, and that the light we see in the Air may be onely patterns taken from the real figure of the light of the Sun: And therefore, if it be according to the former opinion, to wit, That the