Modes of perspective.
If you want to make a figure in a corner of a room that appears made on a flat place, have a nude stripped and by the light of a candle make it cast its shadow to your liking in that corner, and trace its profile with charcoal. But your sight must be exactly in the place, through a hole set where the light passed; and the light of the window behind the work must also come along that line, so that the walls joined together in the corner are not, on account of the shadow, darker one than the other.
Light in the office of perspective has no difference with the eye.
That the light has no difference with the eye as to losing the thing that is behind the first object, the reason is this. You know that through swiftness of motion and concourse of straight lines the visual ray and the luminous ray are alike. For example: you will set a coin close to your eye; that space which lies between the coin and the end of the site, the greater the interval it can span, the greater will be that part of the end of the site which is not seen by the eye, and the nearer the coin approaches the eye, the more of the end of the site will be occupied.
On light.
On the eye.
The same will happen with the light, for in bringing that coin near to or far from the light you will see the shadow grow or diminish on the opposite wall. And if you want the example, do it in this form. Have many bodies of various things placed in a hall, then hold in your hand a long pole with a charcoal at its tip and with it mark on the ground and along the walls all the edges of the walls that appear beyond the edges of the shadows.
On light.
Then at that same distance and height set a light, and you will see the shadows of the said bodies occupy as much of the wall as was that part which was found enclosed within the marks made by the charcoal set at the tip of the pole.
Experiment.
If you want to see such an experiment well, set a light at the head of a table, then move somewhat away and you will see all the shadows of the objects that are between the walls and the light remain stamped as shadows of the form of the objects, and all the lines of their lengths directed to the point where the light is.
Then bring your eye close to that light, making for yourself with the flat of a knife such a shade that the light does not offend the eye, and you will see all the bodies opposite without shadow, and the shadows that were on the walls occupied to the eye by the bodies set in front.