[84] a. Of drawing a thing. See that when you draw, and you move some beginning of a line, you look throughout the whole body you are drawing at whatever thing meets the straight course of the begun line.

[84 a] Book B, folio 17. Note in your drawing how among the shadows there are shadows imperceptible in darkness and in figure; and this is proved by the 5th [proposition], which says: globular surfaces are of as many various darknesses and brightnesses as are the varieties of the darknesses and brightnesses that stand before them as their object.

[85] How high the light for drawing from nature should be. The light for drawing from nature should be from the north, so that it makes no change; and if you use it at midday, keep the window screened with cloth, so that the sun, illuminating all day, may not cause change in it. The height of the light should be so placed that every body casts its shadow along the ground as long as its own height.

[86] What lights should be chosen to draw the figures of bodies. The figures of any body compel you to take that light in which you feign those figures to be: that is, if you feign such figures in the open country, they are girded by a great sum of light, the sun not being uncovered; and if the sun sees the said figures, their shadows will be very dark in respect to the illuminated parts, and will be shadows with sharp boundaries, both the primitive and the derivative; and such shadows will be little companions to the lights, because from one side the blue of the air illuminates and tinges with itself that part which it sees;