Page:The sidereal messenger of Galileo Galilei.pdf/51

 apparently larger than it really is, and would be able to stop our sight from penetrating to the solid body of the Moon, if its thickness were greater; now, it is of greater thickness about the circumference of the Moon, greater, I mean, not in actual thickness, but with reference to our sight-rays, which cut it obliquely; and so it may stop our vision, especially when it is in a state of brightness, and may conceal the true circumference of the Moon on the side towards the Sun.

This may be understood more clearly from the adjoining figure, in which the body of the Moon, ,



is surrounded by an enveloping atmosphere,. An eye at penetrates to the middle parts of the Moon, as at, through a thickness, , of the atmosphere; but towards the extreme parts a mass of atmosphere of greater depth, , shuts out its boundary from our sight. An argument in favour of this