The straight edges of a body will appear broken when they are conterminous with a dark space streaked with rays of light. [Footnote: Here again the diagrams in the original have no connection with the text.] 254. Of several bodies, all equally large and equally distant, that which is most brightly illuminated will appear to the eye nearest and largest. [Footnote: Here again the diagrams in the original have no connection with the text.] 255. If several luminous bodies are seen from a great distance although they are really separate they will appear united as one body. 256. If several objects in shadow, standing very close together, are seen against a bright background they will appear separated by wide intervals. 257. Of several bodies of equal size and tone, that which is farthest will appear the lightest and smallest. 258. Of several objects equal in size, brightness of background and length that which has the flattest surface will look the largest. A bar of iron equally thick throughout and of which half is red hot, affords an example, for the red hot part looks thicker than the rest. 259. Of several bodies of equal size and length, and alike in form and in depth of shade, that will appear smallest which is surrounded by the most luminous background. 260. DIFFERENT PORTIONS OF A WALL SURFACE WILL BE DARKER OR BRIGHTER IN PROPORTION AS THE LIGHT OR SHADOW FALLS ON THEM AT A LARGER ANGLE. The foregoing proposition can be clearly proved in this way. Let us say that _m q_ is the luminous body, then _f g_ will be the opaque body; and let _a e_ be the above-mentioned plane on which the said angles fall, showing [plainly] the nature and character of their bases. Then: _a_ will be more luminous than _b_; the base of the angle _a_ is larger than that of _b_ and it therefore makes a greater angle which will be _a m q_; and the pyramid _b p m_ will be narrower and _m o c_ will be still finer, and so on by degrees, in proportion as they are nearer to _e_, the pyramids will become narrower and darker. That portion of the wall will be the darkest where the breadth of the pyramid of shadow is greater than the breadth of the pyramid of light. At the point _a_ the pyramid of light is equal in strength to the pyramid of shadow, because the base _f g_ is equal to the base _r f_. At the point _d_ the pyramid of light is narrower than the pyramid of shadow by so much as the base _s f_ is less than the base _f g_. Divide the foregoing proposition into two diagrams, one with the pyramids of light and shadow, the other with the pyramids of light [only]. 261. Among shadows of equal depth those which are nearest to the eye will look least deep. 262. The more brilliant the light given by a luminous body, the deeper will the shadows be cast by the objects it illuminates. _V._ _Theory of colours._