17. Now two colours in juxtaposition, _o_ and _p_, will differ from each other in the greatest possible degree when the complementary of _o_ is added to _p_, and the complementary of _p_ is added to _o_; indeed by the juxtaposition of _o_ and _p_, the rays of the colour _p_, which _o_ reflects when it is seen alone, and which are active in that case, cease to be so when _o_ and _p_ are in juxtaposition. Now under these circumstances, each of the two colours, losing what it has analogous to the other, must be so much more different from it. 18. The following formulæ will illustrate this:— Let us represent— The colour of the stripe O by the colour _a plus_ white B, ” ” P ” _aʹ plus_ white Bʹ, the complementary colour of _a_ by C, ” ” _aʹ_” _cʹ_, the colours of the two stripes seen separately are— Colour of O = _a_ + B; colour of P = _aʹ_ + Bʹ; by juxtaposition they become— Colour of O = _a_ + B + _cʹ_, P = _aʹ_ + Bʹ + _c_. We will now show that this expression amounts to taking away the rays of _aʹ_ from the colour _a_ of O (15), and to taking away the rays of the colour _a_ from _aʹ_ of P. For let us suppose— B reduced into two portions, white = _b_ + white = (_a_ʹ + _c_ʹ), Bʹ reduced into two portions, white = _b_ʹ + white = (_a_ + _c_). The colours of the two stripes seen separately are— The colour of O = _a_ + _b_ + _a_ʹ + _c_ʹ, and the colour of P = _a_ʹ + _b_ʹ + _a_ + _c_. By juxtaposition they become— The colour of O = _a_ + _b_ + _c_ʹ, and the colour of P = _a_ʹ + _b_ʹ + _c_. An expression which is evidently the same as the former, except for the values of B and Bʹ. 19. I have said that simultaneous contrast may at the same time affect the optical composition of colours, and the depth of their tone; consequently, when colours are not of the same depth, that which is deep appears deeper, and that which is light appears lighter; that is to say, the former appears to lose white light, while the latter seems to reflect more of it. Thus _there may be, in looking at two contiguous colours, simultaneous contrast of colours and simultaneous contrast of tone_. CHAPTER III. _The Law of Simultaneous Contrast of Colours demonstrated by the Juxtaposition of a certain Number of Coloured Bodies._ 20. Let us now apply the above formula to the seventeen observations of Chapter I., and we shall see that the modifications of contiguous colours are precisely those which would result from the addition to each of them of the complementary of the contiguous colour (18). The rank these colours occupy in Chapter I. may be readily noted, as I have attached to each the number applied to it in that chapter. And for the complementary of each colour see (6). (See Plate II.)