The text explains how the camera obscura can accurately represent objects by using a small opening with a convex glass that inverts the image. It describes how light rays passing through the glass provide true representations of objects on a plane. It also notes that mirrors reflecting these rays do not distort the representation.
METHOD FOR LEARNING
To represent objects in this way, a small opening is made on the side of the dark chamber; a convex glass is placed in this opening, and at the focal point of this glass a paper is stretched on which the objects then appear inverted.
2. The camera obscura provides the true perspective of the objects.
The figures represented in the camera obscura are formed by rays that, coming from all points of the objects, pass through the center of the glass; so that an eye placed at this center would see the objects in these same rays, and consequently they must give the true representation of the objects by their meeting with a plane. But the pyramid formed by these rays outside the chamber is similar to the one they form after passing through the glass, resulting in the rays, which meet the paper in the chamber, also giving the true representation of the objects. C. Q. F. D.
These objects appear inverted because the rays cross as they pass through the glass, those coming from above passing below; and so with the others.
3. The reflection of the rays on a flat mirror, before meeting the convex glass, does not damage the representation of the objects.
This is clear; for the mirror reflects the rays in the same order in which it receives them.
To now demonstrate the use that can be derived from the camera obscura for Drawing, the description of the two machines previously discussed will be given here.