38. It is evident that, all other things being equal, the modifications of contiguous colours will be so much the more marked, as the complementary colour C or Cʹ, which is added to each of them, differs more from them; for the complementary Cʹ which is added to the colour O, is identical with it, as the complementary C is identical with the colour P to which it is added; and the modifications of O and P will simply augment the intensity of their colours. But are there yet any two bodies known which present to the observer two pure colours perfectly complementary to each other? Certainly not; all those which are coloured by reflection reflect (7), besides white light, a great number of rays of various colours. We cannot instance a _red_ body and a _green_ body, or an _orange_ and a _blue_, or an _orange-yellow_ and an _indigo_, or a greenish-yellow and a _violet_, which reflect pure or mixed colours, absolutely complementary to each other. So that the juxtaposition of these colours produces only a simple _augmentation of their intensity_. Hence, although it is less easy to verify the law of contrast with respect to red and green, or orange and blue, &c., than with respect to those which are the object of the seventeen experiments just described (15), yet, upon applying it to the former, it will be seen that their colours acquire a most remarkable brilliancy, strength, and purity. This result, perfectly conformable to the law, may be easily understood: for example, an orange-coloured object reflects blue rays, as a blue object reflects orange rays (7). Hence, when a blue stripe is put in contact with an orange stripe, although it is admitted that the first appears to the eye to receive blue from the orange of the second, as this appears to receive orange from the blue of the blue stripe; or what is the same thing, that the blue stripe appears to destroy the effect of the blue rays of the second stripe, as these appear to destroy the effect of the orange rays of the blue stripe—it is evident that the two colours so contrasted must purify each other, and become more intense. But the blue may incline to green or violet, and the orange to yellow or red; that is to say, the modification may not only affect the intensity of the colour, but also its physical composition. However, if this latter effect take place, it is always much more feeble than the first. Besides, if you look several times at the same coloured stripes, you will see that the blue which at first had appeared to you greenish will afterwards appear inclining to violet; and that the orange, which had appeared at first yellowish, will incline to red; so that the phenomena of modification, as it affects the physical composition of colour, will not have the constancy of those which are the subject of the preceding seventeen observations (15). I now proceed to state the observations I have made on colours which are most nearly complementary to each other. _Red and Green._