112. Then, besides this faculty of clear vision, you have to consider the faculty of metric vision. Neither an eagle, nor a kingfisher, nor any other darting bird, can see things with both their eyes at the same time as completely as you and I can; but think of their faculty of measurement as compared with ours! You will find that it takes you months of labour before you can acquire accurate power, even of _deliberate_ estimate of distances with the eye; it is one of the points to which, most of all, I have to direct your work. And the curious thing is that, given the degree of practice, you will measure ill or well with the eye in proportion to the quantity of life in you. No one can measure with a glance, when they are tired. Only the other day I got half an inch out of a foot, in drawing merely a coat of arms, because I was tired. But fancy what would happen to a swallow, if _it_ was half an inch out in a foot, in flying round a corner! 113. Well, that is the first branch of the questions which we want answered by optical science;--the actual distortion, contraction, and other modification, of the sight of different animals, as far as it can be known from the forms of their eyes. Then, secondly, we ourselves need to be taught the connection of the sense of colour with health; the difference in the physical conditions which lead us to seek for gloom, or brightness of hue; and the nature of purity in colour, first in the object seen, and then in the eye which prefers it. * * * * * (The portion of lecture here omitted referred to illustrations of vulgarity and delicacy in colour, showing that the vulgar colours, even when they seemed most glaring, were in reality impure and dull; and destroyed each other by contention; while noble colour, intensely bright and pure, was nevertheless entirely governed and calm, so that every colour bettered and aided all the rest.) 114. You recollect how I urged you in my opening course of lectures rather to work in the school of crystalline colour than in that of shade. Since I gave that first course of lectures, my sense of the necessity of this study of brightness primarily, and of purity and gaiety beyond all other qualities, has deeply been confirmed by the influence which the unclean horror and impious melancholy of the modern French school--most literally the school of death--has gained over the popular mind. I will not dwell upon the evil phrenzy to-day. But it is in order, at once to do the best I can, in counteraction of its deadly influence, though not without other and constant reasons, that I give you heraldry, with all its splendour and its pride, its brightness of colour, and honourableness of meaning, for your main elementary practice. 115. To-day I have only time left to press on your thoughts the deeper law of this due joy in colour and light.