The example we have taken, teaches us several good and important lessons. There is in it much we can understand; much that we cannot understand; and a twilight territory between the intelligible and the non-intelligible. All three are, of course, mixed together when we speak of sight,--in itself, a matter of every-day experience. So far as the mechanical construction of an optical chamber goes, everything seems obvious. We can, likewise, perceive how well contrived is the apparatus for washing and wiping the outside transparent surface. Also, the value of its arched hedge against irritants dropping upon the eyeball from above; and of the arrangements for altering both axis and focus instantaneously. But what does this instrument enable us to see? _Not_ the rays of light themselves,--only objects which they illuminate. The space traversed by rays from all suns and all stars, remains itself unseen. The ether which fills space is invisible,--yet its motions make the light of the world.[93] Then, too, the nervous screen on which these ray movements are received, is not sensitive to all transmitted undulations. Red excites the optic nerve by striking it with four hundred and seventy-four _millions of millions_ of wave-impacts in a single second. Violet strikes it in the same time with six hundred and ninety-nine _millions of millions_ of impulses.[94] These two colours are the extremes of the light octave. In an octave of sound, the highest note vibrates twice as quickly as the lowest. So too, the shortest wave of violet is half the length of the longest red wave, and its motion is twice as rapid. But the curious point is that the human ear receives eleven octaves in the scale of sound;[95]--the human eye has a range over only one octave in the scale of light. Our remarks have carried us over the borders of the twilight territory,--a circumstance we may ascertain by putting into words what we think we know, and our reasons for thinking that we know it. If the eye be in focus, (but not otherwise), a line of light--that is to say moving imponderable matter of extreme tenuity--so passes through its transparent liquids as to strike a sensitive spot, and there produce what is called an image. We apprehend in our minds this image-producing function as a relation between light and the effect realized. A relation definite and exact,--in scientific language a "constant"; which we can formulate into optical laws, and thus express with useful nicety. Taking advantage of the laws thus obtained, and employing that light-power which everywhere blesses our world, we reproduce the _like_, image upon a screen. Its likeness we gather from comparison, by looking into an eye from without. Both images, thus seen by us, are in point of fact similar sensations.