The eye f and the eye t are one and the same thing; but the eye f marks the distance, that is to say how far you are standing from the object; and the eye t shows you the direction of it; that is whether you are opposite, or on one side, or at an angle to the object you are looking at.  And remember that the eye f and the eye t must always be kept on the same level.  For example if you raise or lower the eye from the distance point f you must do the same with the direction point t.  And if the point f shows how far the eye is distant from the square plane but does not show on which side it is placed—­and, if in the same way, the point t show s the direction and not the distance, in order to ascertain both you must use both points and they will be one and the same thing.  If the eye f could see a perfect square of which all the sides were equal to the distance between s and c, and if at the nearest end of the side towards the eye a pole were placed, or some other straight object, set up by a perpendicular line as shown at r s—­then, I say, that if you were to look at the side of the square that is nearest to you it will appear at the bottom of the vertical plane r s, and then look at the farther side and it would appear to you at the height of the point n on the vertical plane.  Thus, by this example, you can understand that if the eye is above a number of objects all placed on the same level, one beyond another, the more remote they are the higher they will seem, up to the level of the eye, but no higher; because objects placed upon the level on which your feet stand, so long as it is flat—­even if it be extended into infinity—­would never be seen above the eye; since the eye has in itself the point towards which all the cones tend and converge which convey the images of the objects to the eye.  And this point always coincides with the point of diminution which is the extreme of all we can see.  And from the base line of the first pyramid as far as the diminishing point

[Footnote:  The two diagrams above the chapter are explained by the first five lines.  They have, however, more letters than are referred to in the text, a circumstance we frequently find occasion to remark.]

56.

there are only bases without pyramids which constantly diminish up to this point.  And from the first base where the vertical plane is placed towards the point in the eye there will be only pyramids without bases; as shown in the example given above.  Now, let a b be the said vertical plane and r the point of the pyramid terminating in the eye, and n the point of diminution which is always in a straight line opposite the eye and always moves as the eye moves—­just as when a rod is moved its shadow moves, and moves with it, precisely as the shadow moves with a body.  And each point is the apex of a pyramid, all having a common base