273. 2. _If the stuff is orange_, A is more orange or less grey than B; and the portion _a_ is deeper, more vivid than _aʹ_, as _b_ is more grey and duller than _bʹ_. 274. 3. _If the stuff is yellow_, A is more vivid, more orange than B; _a_ is more so than _aʹ_, as _b_ is duller than _bʹ_. 275. 4. _If the stuff is green_, A is less blue or more yellow than B; and _a_ is of a yellower green than _aʹ_, as _b_ is bluer than _bʹ_. 276. 5. _If the stuff is blue_, A is less violet and more green than B; and _a_ is greener than _aʹ_, as _b_ is more violet or less green than _bʹ_. 277. 6. _If the stuff is indigo_, A is redder or less blue than B; and _a_ is redder than _aʹ_, as _b_ is deeper or bluer than _bʹ_. 278. 7. If the stuff is violet, A is less blue than B; and _a_ is redder than _aʹ_ as _b_ is bluer than _bʹ_. 279. 2nd. Two contiguous parts of the same object unequally illuminated by the same light, when viewed simultaneously, differ from each other, not only in depth of tone, but also in optical composition of colour. Place half-a-sheet of coloured paper (Plate I., fig. 5) upon the partition _b_, of a chamber receiving diffused daylight by a window _f_: place another half-sheet upon the partition _a_, in such a manner that it will be lighted directly by the diffused light, while the other is only indirectly lighted by reflection from the walls, floor, and ceiling: the diffused light thus reflected being only white light, then stand at _c_, so as to see both half-sheets at once. I shall designate that which is upon the partition _a_, and most lighted, by A, and the other, which is upon the partition _b_, and less lighted, by B. These letters in the plate indicate the respective positions of the half-sheets. 280. The inference from these observations is, that the colour of the same body varies, not only in intensity of tone, but also of hue, according as it is lighted directly by the sun, by diffused daylight, or by diffused reflected light. This result must never be overlooked whenever we define the colours of material objects. 3rd. _Modifications produced by diffused daylight reflected by a surface all the parts of which are not in the same position relatively to the eye of the spectator._ 281. Distant bodies are rendered perceptible to the eye only in proportion as they radiate or reflect, or transmit the light which acts upon the retina. According to the laws of reflection, it happens that those portions of a surface which are in relief, or hollow, must reflect the light in such a manner, that the eye of the spectator, in a given position, will see these parts very variously lighted, in respect to the intensity of reflected light, so that the parts of this surface will be, relatively to the eye, in the same condition as the homogeneous parts of a plane surface, which are illuminated by lights of unequal intensity.