It is probably in birds that vision reaches its maximum of acuteness. A tame jackdaw will show signs of uneasiness when seemingly nothing is visible in the sky. Presently, far up, a mere speck in the blue, a hawk will come within the range of far-sighted human vision. Steadily watch the speck as the hawk soars past, until it ceases to be visible; the jackdaw will still keep casting his eye anxiously upward for some little time. He may be only watching for the possible reappearance of the hawk. But just as he saw it before man could see it, so probably he still watches it after, to man's sight, it has become invisible. So, too, for nearer minute objects, the swift, as it wheels through the summer air, presumably sees the minute insects which constitute its food. And every one must have noticed how domestic fowls will pick out from among the sand-grains almost infinitesimal crumbs. It is probable that the area of acute vision is much more widely diffused over the retina of birds than it is with us. In any case, the cones are more uniformly and more abundantly distributed over the general retinal surface. An exceedingly interesting and important peculiarity in the retina of birds, which they share with some reptiles and fishes, is the development, in the cones, of coloured globules. "The retinæ of many birds, especially of the finch, the pigeon, and the domestic fowl, have been carefully examined by Dr. Waelchli, who finds that near the centre green is the predominant colour of the cones, while among the green cones red and orange ones are somewhat sparingly interspersed, and are nearly always arranged alternately--a red cone between two orange ones, and _vice versâ_. In a surrounding portion, called by Dr. Waelchli the red zone, the red and orange cones are arranged in chains, and are larger and more numerous than near the yellow spot; the green ones are of smaller size, and fill up the interspaces. Near the periphery the cones are scattered, the three colours about equally numerous and of equal size, while a few colourless cones are also seen. Dr. Waelchli examined the optical properties of the coloured cones by means of the micro-spectroscope, and found, as the colours would lead us to suppose, that they transmitted only the corresponding portions of the spectrum; and it would almost seem, excepting for the few colourless cones at the peripheral part of the retina, that the birds examined must have been unable to see blue, the whole of which would be absorbed by their colour-globules."[FH]