Circle 



4.r 4 



390 DR. J. C. MOTTEAM ON 



Circumference 

 Area 



2V''^-P _3-56 



Square 



(2 + 2V'2)a.-_4-83 

 Isosceles triangle of 90 p Z^' 



^ , , Sx 5 



Eectaugle,2Xi ;^;;2 = -• 



T. 1-1 8'5a; 8'5 



Kectansle, 4X3 - o = -. 



"" a" a; 



In text-fig. 4 the inverse visibility figures (X) are conventionally plotted with the 

 numerator of the periphery over area (®). It can be seen that the visibility 

 curve closely follows the concentration of area curve. 



the surrounding natural backgrounds : it will be foiind that it 

 is very much lighter in tone than the lightest natural back- 

 ground which can be found in the wood. 



On referring to the diagram (text-fig. 2, p. 386) it can be 

 seen that the difference in visibility between white and black 

 is greater at high illumination than at low. It follows that 

 at night white is, in Nature, not nearly so conspicuous with 

 regard to black as it is during daylight. 



Diurnal animals presenting a large area of white or light tone 

 must therefore be considered to be much more conspicuous than 

 nocturnal animals similarly pattei-ned. 



The Effect of the Object's Characters on Visibility. — The cha- 

 racters, size, and shape will affect the visibility of an object 

 when all other factors are kept constant. When the contrast 

 in tone between object and background is great (the object 

 being light in tone and the background dark), the human eye 

 is able to define an object subtending an angle of approximately 

 one minute. Distinction must be made between the ability to 

 define or focus, and that to see : the eye cannot define a star, 

 although it may be able to see it. 



Keeping the shape of the object constant, experiments show 

 that visibility is directly proportional to size : the larger the 

 object, the greater the distance at which it is visible. 



\\ hether the relation between size and visibility is the same 

 for all shapes has not been determined ; though during the 

 course of this and other investigations a lai-ge number of 

 shapes have been examined, no exceptions have been noted. 



The visibility of objects is dependent upon their shape. Circles, 

 squares, triangles, and rectangles of the same area are not 

 equally visible. Experiments show that the moi'e concentrated 

 the area the greater the visibility. 



In the following experiments concentration is measured by 

 the ratio of circumference over area, and it can be seen that the 

 distance at which the object is visible is inversely proportionate 

 to this ratio (see experiment no. 5). 



