308 SCIENCE FROM AN EASY CHAIR 



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 hand is withdrawn it melts away again like Banquo's 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 con- 

 cealment 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, swim- 



