with the intervening vertical plane.  But although their bases are equal their angles are not equal, because the diminishing point is the termination of a smaller angle than that of the eye.  If you ask me:  “By what practical experience can you show me these points?” I reply—­so far as concerns the diminishing point which moves with you —­when you walk by a ploughed field look at the straight furrows which come down with their ends to the path where you are walking, and you will see that each pair of furrows will look as though they tried to get nearer and meet at the [farther] end.

[Footnote:  For the easier understanding of the diagram and of its connection with the preceding I may here remark that the square plane shown above in profile by the line c s is here indicated by e d o p.  According to lines 1, 3 a b must be imagined as a plane of glass placed perpendicularly at o p.]

57.

How to measure the pyramid of vision.

As regards the point in the eye; it is made more intelligible by this:  If you look into the eye of another person you will see your own image.  Now imagine 2 lines starting from your ears and going to the ears of that image which you see in the other man’s eye; you will understand that these lines converge in such a way that they would meet in a point a little way beyond your own image mirrored in the eye.  And if you want to measure the diminution of the pyramid in the air which occupies the space between the object seen and the eye, you must do it according to the diagram figured below.  Let m n be a tower, and e f a, rod, which you must move backwards and forwards till its ends correspond with those of the tower [Footnote 9:  I sua stremi .. della storre (its ends ... of the tower) this is the case at e f.]; then bring it nearer to the eye, at c d and you will see that the image of the tower seems smaller, as at r o.  Then [again] bring it closer to the eye and you will see the rod project far beyond the image of the tower from a to b and from t to b, and so you will discern that, a little farther within, the lines must converge in a point.

The Production of pyramid of Vision (58-60).

58.

PERSPECTIVE.

The instant the atmosphere is illuminated it will be filled with an infinite number of images which are produced by the various bodies and colours assembled in it.  And the eye is the target, a loadstone, of these images.

59.

The whole surface of opaque bodies displays its whole image in all the illuminated atmosphere which surrounds them on all sides.

60.

That the atmosphere attracts to itself, like a loadstone, all the images of the objects that exist in it, and not their forms merely but their nature may be clearly seen by the sun, which is a hot and luminous body.  All the atmosphere, which is the all-pervading matter, absorbs light and heat, and reflects in itself the image of the source of that heat and splendour and, in each minutest portion, does the same.  The Northpole does the same as the loadstone shows; and the moon and the other planets, without suffering any diminution, do the same.  Among terrestrial things musk does the same and other perfumes.