Our author next refers to a rather singular subject, which has not yet sufficiently attracted the notice of naturalists, and the phenomena of which (at least their final causes) have not been explained by physiological enquirers. That fishes assume, in a great degree, the colour of the channel over which they lie, is known to many practical observers. We have ourselves frequently frightened small flounders from their propriety with our shoe-points, while angling near the mouths of rivers, and so exactly did their colour accord with the shingle beneath our feet, that we could not detect their presence but by their own betraying movements. Such, however, as happened to glide towards, and settle on, a portion of the bed of different colour from the rest, continued perceptible for a short time; but they too seemed speedily to disappear, although we afterwards discovered that they had not stirred an inch, but had merely changed their tint to that of the particular portion of the basin of the stream to which they had removed. Every angler knows, that there is not only a difference in the colour of trouts in different streams, but that different though almost adjoining portions of the same river, if distinguished by some diversity of character in respect to depth, current, or clearness, will yield him fish of varying hue. Very rapid and irregular changes are also observable in their colours after death; and large alternate blotches of darker and lighter hues may be produced upon their sides and general surface, by the mode of their disposal in the creel. Dr Stark showed many years ago, that the colour of sticklebacks, and other small fishes, was influenced by the colour of the earthenware, or other vessels in which they were confined, as well as modified by the quantity of light to which they were exposed; and Mr Shaw has very recently informed us, regarding this mutability of the outer aspect of fishes, that if the head alone is placed upon a particular colour, (whether lighter or darker,) the whole body will immediately assume a corresponding shade, quite independent of the particular tint upon which the body itself may chance to rest. We know not to what extent these, and similar phenomena, are familiar to Sir David Brewster; but we willingly admit, that in order to attain to their clearer comprehension, the facts themselves must be investigated by one who, like that accomplished philosopher, is conversant with those branches of physical science to which they are related. They unfortunately lie beyond the range of our own optics, but Mr Scrope's practical improvement of the subject is as follows:--