_B._ There are divers causes that take away transparency. First, if the body be not perfectly homogeneous, that is to say, if the smallest parts of it be not all precisely of the same nature, or do not so touch one another as to leave no vacuum within it; or though they touch, if they be not as hard in the contact as in any other line. For then the refractions will be so changed both in their direction, and in their strength, as that no light shall come through it to the eye; as in wood and ordinary stone and metal. Secondly, the gravity and hardness may be so great, as to make the angle refracted so great, as the second refraction shall not direct the beam of light to the eye; as if the angle of refraction were D A E, the refracted line would be perpendicular to A B, and never come to the line A D, in which is the eye. _A._ To know how much of the refraction is due to the hardness, and how much to the gravity, I believe it is impossible, though the quantity of the whole be easily measured in a diaphanous body given. And both you and Mr. Warner have demonstrated, that as the sine of the angle refracted in one inclination is to the sine of the angle refracted in another inclination, so is the sine of one inclination to the sine of the angle of the other inclination. Which demonstrations are both published by Mersennus in the end of the first volume of his _Cogitata Physico-Mathematica_. But since there be many bodies, through which though there pass light enough, yet no object appear through them to the eye, what is the reason of that? _B._ You mean paper. For paper windows will enlighten a room, and yet not show the image of an object without the room. But it is because there are in paper abundance of pores, through which the air passing moveth the air within; by the reflections whereof anything within may be seen. And in the same paper there are again as many parts not transparent, through which the air cannot pass, but must be reflected first to all parts of the object, and from them again to the paper; and at the paper either reflected again or transmitted, according as it falls upon pores or not pores; so that the light from the object can never come together at the eye. _A._ There belongs yet to this subject the causes of the diversity of colours. But I am so well satisfied with that which you have written of it in the twenty-fourth chapter of your book _de Corpore_, that I need not trouble you farther in it. And now I have but one question more to ask you, which I thought upon last night. I have read in an ancient historian, that living creatures after a great deluge were produced by the earth, which being then very soft, there were bred in it, it may be by the rapid motion of the sun, many blisters, which in time breaking, brought forth, like so many eggs, all manner of living creatures great and small, which since it is grown hard it cannot do. What think you of it?