The text provides instructions on using mirrors to determine angles for inclined and parallel paintings. It includes specific instructions for positioning mirrors to achieve precise angles relative to the horizon. The method is applicable for various angles, relying on a detailed understanding of reflection and inclination.
146 METHOD FOR LEARNING
the top of the machine, after a double reflection, appears vertical in the first mirror.
20. For an inclined painting, mirror L must form with the horizon an angle equal to half the angle of the painting's inclination, minus a quarter of a right angle. This angle is found with sufficient precision for practice by tilting mirror L until the appearance of the top of the machine, after a double reflection, appears in the other mirror at an angle with the horizon equal to the inclination you want to give to the painting. If the inclination of the painting is less than a quarter of 90 degrees, do not tilt mirror L toward the first mirror as stated (19), but towards the opposite side, making the inclination angle on the mirror equal to the difference between the inclination angle of the painting and a quarter of 90 degrees.
21. When you want to represent objects for a parallel painting, place mirror L in the arrangement indicated in (No. 15.) and mirror H in the one outlined (No. 10.) by tilting it towards the horizon under a semi-right angle, with the glass turned towards the ground when you assume the painting is below the eye and towards the sky when it is assumed above.
22. This machine configuration can also be used for inclined paintings, which are at a very small angle with the horizon; however, you should reduce the inclination of one of the mirrors by half of the painting's inclination.