[Footnote:  81.  On the original diagram at the beginning of this chapter Leonardo has written “azurro” (blue) where in the facsimile I have marked A, and “giallo” (yellow) where B stands.]

[Footnote:  15—­23.  These lines stand between the diagrams I and III.]

[Footnote:  24—­53.  These lines stand between the diagrams I and II.]

[Footnote:  54—­97 are written along the left side of diagram I.]

82.

An experiment showing that though the pupil may not be moved from its position the objects seen by it may appear to move from their places.

If you look at an object at some distance from you and which is below the eye, and fix both your eyes upon it and with one hand firmly hold the upper lid open while with the other you push up the under lid—­still keeping your eyes fixed on the object gazed at—­you will see that object double; one [image] remaining steady, and the other moving in a contrary direction to the pressure of your finger on the lower eyelid.  How false the opinion is of those who say that this happens because the pupil of the eye is displaced from its position.

How the above mentioned facts prove that the pupil acts upside down in seeing.

[Footnote:  82. 14—­17.  The subject indicated by these two headings is fully discussed in the two chapters that follow them in the original; but it did not seem to me appropriate to include them here.]

Demostration of perspective by means of a vertical glass plane (83-85).

83.

OF THE PLANE OF GLASS.

Perspective is nothing else than seeing place [or objects] behind a plane of glass, quite transparent, on the surface of which the objects behind that glass are to be drawn.  These can be traced in pyramids to the point in the eye, and these pyramids are intersected on the glass plane.

84.

Pictorial perspective can never make an object at the same distance, look of the same size as it appears to the eye.  You see that the apex of the pyramid f c d is as far from the object c d as the same point f is from the object a b; and yet c d, which is the base made by the painter’s point, is smaller than a b which is the base of the lines from the objects converging in the eye and refracted at s t, the surface of the eye.  This may be proved by experiment, by the lines of vision and then by the lines of the painter’s plumbline by cutting the real lines of vision on one and the same plane and measuring on it one and the same object.

85.

PERSPECTIVE.

The vertical plane is a perpendicular line, imagined as in front of the central point where the apex of the pyramids converge.  And this plane bears the same relation to this point as a plane of glass would, through which you might see the various objects and draw them on it.  And the objects thus drawn would be smaller than the originals, in proportion as the distance between the glass and the eye was smaller than that between the glass and the objects.