I say that the power of vision extends through the visual rays to the surface of non-transparent bodies, while the power possessed by these bodies extends to the power of vision. Likewise each body pervades the surrounding air with its image; each separately and all together do the same; and not only do they pervade it with the semblance of the shape, but also with that of their power.
Example
You will see when the sun is over the centre of our hemisphere that wherever it reveals itself there are semblances of form; and you will also perceive the reflections of its radiance as well as the glow of its heat; and all these powers proceed from the same source by means of radiant lines that issue from its body and they end in the opaque objects without entailing any diminution at the source.
Confutation
Those mathematicians then who argue that no spiritual power can emanate from the eye, because this could not be without greatly impairing the power of vision, and therefore maintain that the eye takes in but does not send forth anything from itself.
Example
What will they say of the musk which always keeps a great quantity of its surrounding atmosphere charged with odour, and which when carried miles will permeate a thousand miles with that perfume without diminution of itself?
Or will they say that the ringing of the bell by its clapper, which daily fills the whole countryside with its sound, must of necessity consume this bell?
Certainly it seems, there are such men as these—but enough! Is not that snake called lamia seen daily by the rustics attracting to itself with fixed gaze as the magnet attracts iron, the nightingale which hastens to her death with mournful song? . . . Maidens are said to have power in their eyes to attract to themselves the love of men. . . .
15
(
c
) Perspective
Perspective is the bridle and rudder of painting.
16
Painting is based upon perspective which is nothing else than a thorough knowledge of the function of the eye. And this function simply consists in receiving in a pyramid the forms and colours of all objects placed before it. I say in a pyramid, because there is no object so small that it will not be larger than the spot where these pyramids are received into the eye. Therefore if you extend the lines from the edges of each body as they converge you will bring them to a single point, and necessarily the said lines must form a pyramid.
17
There are three branches of perspective; the first deals with the reasons of the (apparent) diminution of objects as they recede from the eye, and is known as Perspective of Diminution; the second contains the way in which colours vary as they recede from the eye; the third and last explains how objects should appear less distinct in proportion as they are more remote. And the names are as follows: Linear perspective, the perspective of colour, the perspective of disappearance.*
18
The science of painting deals with all the colours of the surfaces of bodies and with the shapes of the bodies thus enclosed; with their relative nearness and distance; with the degrees of diminution required as distances gradually increase; and this science is the mother of perspective, that is the science of visual rays. Perspective is divided into three parts, of which the first deals only with the line-drawing of bodies; the second with the toning down of colours as they recede into the distance; the third with the loss of distinctness of bodies at various distances. Now the first part which deals only with lines and boundaries of bodies is called drawing, that is to say the figuration of any body. From it springs another science that deals with shade and light, also called chiaroscuro which requires much explanation.
19
Perspective is nothing else than the seeing a place behind a sheet of glass, smooth and quite transparent, on the surface of which all the things may be marked that are behind this glass. The things approach the point of the eye in pyramids, and these pyramids are intersected on the glass plane.
20
A method of drawing an object in relief at night
Place a sheet of not too transparent paper between the object and the light and you can draw it very well.
20
Every bodily form, so far as it affects the eye, includes three attributes; namely mass, shape, and colour; and the mass is recognizable at a greater distance from its source than either colour or shape. Again colour is discernible at a greater distance than shape, but this law does not apply to luminous bodies.
21
Perspective
Among objects of equal size that which is most remote from the eye will look smallest.
22
Of several bodies of equal size and tone, that which is furthest will appear lightest and smallest.
23
Of several bodies, all equally large and equally distant, that which is most brightly illuminated will appear to the eye nearest and largest.
24
Among shadows of equal depth those nearest to the eye will look least deep.
25
A dark object will appear bluer in proportion as it has more luminous atmosphere between it and the eye, as may be seen in the colour of the sky.
26
I ask to have this much granted me (as axiom)—to assert that every ray passing through air of equal density travels in a straight line from its cause to the object or place where it strikes.*
27
The air is full of infinite straight and radiating lines intersected and interwoven with one another, without one occupying the place of another. They represent to whatever object the true form of their cause.
28
The body of the atmosphere is full of infinite radiating pyramids produced by the objects existing in it. These intersect and cross each other with independent convergence without interference with each other and pass through all the surrounding atmosphere.
29
The vertical plane is represented by a perpendicular line and it is imagined as being placed in front of the common point where the concourse of the pyramid converges. And this plane bears the same relation to this point as a plane of glass would, upon which you drew the various objects that you saw through it. And the objects thus drawn would be so much smaller than the originals as the space between the glass and the eye was smaller than that between the glass and the objects.
30
If the eye be in the middle of a course with two horses running to their goal along parallel tracks, it will seem to it as if they were running to meet one another. This, as has been stated, occurs because the images of the horses which impress themselves upon the eye are moving towards the centre of the surface of the pupil of the eye.
31
As regards the point in the eye it may be comprehended with greater ease if you look into the eye of anyone you will see your image there. Now imagine two lines starting from your ears and going to the ears of that image which you see of yourself in the eye of the other person. You will clearly recognize 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.
32
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 above. Let
mn
be a tower, and
ef
a rod which you must move backwards and forwards till its ends correspond with those of the tower; then bring it nearer to the eye, at
cd
and you will see that the image of the tower seems smaller as at
to
. Then bring it still closer to the eye, and you will see the rod project far beyond the image of the tower, . . . and so you will discern that, a little further within the lines must converge in a point.
32