288 Letters, Extracts, Notices, ^c. 



Cambridge^ Mass., on Nov. lOth, 1896 (as we learn from 

 'Science'), Mr. Thayer showed the following curious experi- 

 ment. He placed three sweet potatoes, or objects of corre- 

 sponding shape and size, horizontally on a wire a few inches 

 above the ground. They were covered with some sticky 

 material, and dry earth from the road on which they stood 

 was sprinkled over them, so that they would be the same 

 colour as the background. The two end ones were then 

 painted white on the underside, and the white colour 

 was shaded up and gradually mixed with the brown of 

 the sides. When viewed from a little distance, these two 

 end ones, which were white below, disappeared from sight, 

 while the middle one stood out in strong relief, and 

 appeared much darker than it really was. Mr. Thayer 

 explained that terrestrial birds and mammals which are 

 protectively coloured have the underparts white or very 

 light in colour, and that the colour of the underparts 

 usually shades gradually into that of the upper parts. This 

 is essential in order to counteract the effect of the shadow, 

 which otherwise, as shown by the middle potato, makes the 

 object abnormally conspicuous, and causes it to appear much 

 darker than it really is. In the case of Mr. Thayer's 

 experiment, some of the witnesses could hardly believe that 

 the striking difference in the visibility of the three potatoes 

 was entirely due to the colouring of the underside, and 

 Mr. Thayer was asked to colour the middle one like the 

 two others, in order that the effect might be observed. 

 Mr. Thayer complied with the request, painting the under- 

 side of the middle potato white, and shading the white up 

 into the sides, as in the case of the others. The effect was 

 almost magical. The middle potato at once disappeared 

 from view. A similar experiment was tried on the lawn. 

 Two potatoes were painted green, to resemble the green of 

 the grass above which they were suspended. One was 

 painted white on the underside, and at once became 

 invisible when viewed from a little distance, while the 

 other showed plainly and seemed very dark, the shadow, 

 superadded to the green of the underside, making it remark- 



