sunlight, which is not a thing one can always ensure in a London museum) the duck-model so treated is absolutely invisible at a distance of 10 ft. or 15 ft.—and even when one is nearer escapes notice—looking like a haze or vague shadow of a bird even to an observer who knows nevertheless that it is there and is really as solid and large as the untreated model by its side. If now some one stretches out his hand so as to cut off the top-light falling on the painted model, it immediately becomes as solid to the eye as the untreated one, and when the hand is withdrawn it melts away again like Banquo’s ghost. The models made by Mr. Thayer were, so long as I was director, exhibited in the small room between the fish gallery and the central hall of the Natural History Museum, and, if they have not yet been removed, are well worth a visit. Mr. Thayer’s models work perfectly, and astonish every one who sees them. The great point of interest about them, however, is, that the bird with dark back and light belly is really in the condition which is quite common in a number of birds, especially ducks and wading birds, where it must act as a means of concealing the bird—just as it does in the painted model. Of course, there are vast numbers of birds not so shaded, but it is possible to explain the darker and lighter colouring, in various arrangements seen in birds, as helping to produce concealment or disappearance from view, when the habits and natural surroundings of the bird are known. So, too, with many hairy quadrupeds (mammals, or “animals,” or “beasts,” as they are often called). The white hair under the tail and about the rump, helps a running animal to escape the vision of its pursuer—blending, as Mr. Thayer shows that it does—with the white colour of the sky-line. In the case of fish—especially fresh-water fish—the dark back and light belly are very common, and although they do not help to conceal the fish when seen from above, swimming over a light-coloured river-bed, yet when looked at by other fishes or by otters in the water, the effect of the light from above on this disposition of dark and light tints on the fish’s body must be the same as that demonstrated by Mr. Thayer’s “disappearing duck,” and must often render the fish absolutely invisible, even at close quarters.