Light on an Octagon's Faces by the Angle of Incidence
Central lines r-a and r-d, and mixing colour in proportion to the angle
The page continues the rule for lighting a faceted body, referring to an octagonal cross-section drawn in the margin. Central lines r-a and r-d run from the centre of the luminous body r to the centres of sides s-c and c-f; the quantity of light each side receives is in the same proportion as the angles b-a-c and e-d-f. A drawn octagon at the top of the page, with lines converging from a point above it, illustrates this construction. A new chapter [756] then gives a practical recipe: match the body's colour and the light's colour, and if the larger angle is double the smaller, mix two parts of light to one part of the body's colour, so the light is doubled.
On this page
Central lines and angles on the octagonal face
Line r-a extends from the centre of the luminous body r to the centre of side s-c, and line r-d from the same centre to the centre of side c-f. The light each side receives stands in the same proportion as the size of angle b-a-c to angle e-d-f. An octagon with converging construction lines is drawn in the margin.
[756] Recipe for mixing light double the lesser light
Take a colour like that of the body to be imitated and the colour of its principal light. If the greater angle is found to be double the lesser, take one part of the body's natural colour and give it two parts of light, so the light is set double to the lesser light; then, to make the light subduple, take...