_B._ How that should be I cannot imagine. For when all things in the cause are equal, the effects cannot be unequal. As soon as fire is given, and before the bullet be out, the gun begins to recoil. If then there be any unevenness or rub in the ground more on one side than on the other, it shall shoot beside the mark, whether too much, or too little, or justly charged; because if the line wherein the gun recoileth decline, the way of the bullet will also decline to the contrary side of the mark. Therefore I can imagine no cause of this event, but either in the ground it recoils on, or in the unequal weight of the parts of the breech. _A._ How comes refraction? _B._ When the action is in a line perpendicular to the superficies of the body wrought upon, there will be no refraction at all. The action will proceed still in the same straight line, whether it be pression as in light, or percussion as in the shooting of a bullet. But when the pression is oblique, then will the refraction be that way which the nature of the bodies through which the action proceeds shall determine. _A._ How is light refracted? _B._ If it pass through a body of less, into a body of greater resistance, and to the point of the superficies it falleth on, you draw a line perpendicular to the same superficies, the action will proceed not in the same line by which it fell on, but in another line bending toward that perpendicular. _A._ What is the reason of that? _B._ I told you before, that the falling on worketh only in the perpendicular; but as soon as the action proceedeth farther inward than a mere touch, it worketh partly in the perpendicular, and partly forward, and would proceed in the same line in which it fell on, but for the greater resistance which now weakeneth the motion forward, and makes it to incline towards the perpendicular. _A._ In transparent bodies it may be so; but there be bodies through which the light cannot pass at all. _B._ But the action by which light is made, passeth through all bodies. For this action is pression; and whatsoever is pressed, presseth that which is next behind, and so continually. But the cause why there is no light seen through it, is the unevenness of the parts within, whereby the action is by an infinite number of reflections so diverted and weakened, that before it hath proceeded through, it hath not strength left to work upon the eye strongly enough to produce sight. _A._ If the body being transparent, the action proceed quite through, into a body again of less resistance, as out of glass into the air, which way shall it then proceed in the air? _B._ From the point where it goeth forth, draw a perpendicular to the superficies of the glass, the action now freed from the resistance it suffered, will go from that perpendicular, as much as it did before come towards it. _A._ When a bullet from out of the air entereth into a wall of earth, will that also be refracted towards the perpendicular.