080 UR. J. C. MOTTRAM OX 



the case of alternate black and white squares and stripes under 

 the experimental conditions stated. 



Further experiments showed that the blending distance of 

 different shapes depends upon the relative concentration of the 

 black and white areas, the more concentrated the greater the 

 blending distance. If, for instance, a checkered surface composed 

 of squares of 2 square millimetres be compared with a surface 

 composed of black and white oblongs of 1 multiplied by 4 mm. or 

 •5 multiplied by 8 mm. or "025 by 16 mm., as regards the blending 

 distance, it will be found that the distance is greatest for the 

 squai^es and least for the narrowest oblongs. On comparing 

 squares with other figures in this respect, for instance, with circles 

 or triangles, it was found that the blending distance was related 

 to the concentration of the various shapes. 



The Surface lohich Results after Blending. 



If a patterned surface of black and white be gradually 

 approached from a distance, the surface which at first appeared 

 of a perfectly even grey tone will, at a certain point, become gran- 

 ular, and a very short distance in front of this the pattern will 

 suddenly shine forth clear and defined. 



By varying the relative proportion of black to white in the 

 pattern, all tones of grey can be produced when the pattern is 

 blended. It is thus possible to match a given grey surface by 

 means of blnck and white pattern, and also possible to reproduce 

 a, graded surface of grey : text-fig. 3, i, shows without any de- 

 scription how this can be done. If these copies of greys with 

 patterns be fixed to solid figures such as a cylinder and be then 

 examined, it will be found that after blending, the copies are 

 undisturbed. It follows that the obliterative shading so com- 

 monly seen on animals and consisting of low tones where the 

 light strikes, high tones in the shadows, and intermediate tones 

 between the two, could be reproduced by means of pattern. 

 Search has therefore been made amongst animals to discover 

 whether this method exists. 



Some Examples of Animals tvhich appear to shoiv Obliterative 

 Shading hy meoMS of Pattern- Blending. 



Grant's Zebra (^Equvs bur chelli granti). — This animal's cent is 

 coloured black and white, black stripes on a white background : 

 the background, unlike that of the vast majority of animals, pre- 

 sents no obliterative shading, it has the same tone throughout ; 

 the stripes are somewhat dai'ker on the back than they are below. 

 On referring to text-fig. 2. o, it can be seen that on the dorsal 

 sui-face of the animal and on those parts of the body where the 

 light strikes, the stripes are broad, whereas on those parts which 

 luider natural conditions w^ould be in shadow, the stripes are 

 narrow : further, on those areas which would be more or less 



