392 DR. 3. C. MOTTRAM ON 



In experiment no. 6 mathematical expressions are avoided 

 by presenting the concentration as the length of a cord. Areas 

 of the same size and circumference, but of difierent concentration, 

 are dealt with in this experiment ; and the same conclusion is 

 arrived at, namely, that the more concentrated the area of the 

 object the greater its visibility. 



This completes the consideration of plain objects against plain 

 backgrounds. The following facts have been observed : — 



1. The gi-eater the difference in the amount of light coming 



from an object and background, the greater the distance 

 at which the object is visible. 



2. A constant contrast in tone between object and back- 



ground does not ensure a constant visibility. Dark 

 objects against light backgrounds are less visible than 

 light objects against dark backgrounds. This difference 

 is greater at high illuminations than at low ones. 



3. The larger the size of the object, and the greater the 



concentration of its area, the greater the distance at 

 which it is visible. 



It follows that a plain object will appear conspicuous against a 

 plain background when the contrast in tone between object and 

 background is great, and when the object is lighter rather than 

 darker in tone than the background, and when its size is great 

 and area concentrated. 



(B) Patterned Objects against a Plain Background. 



As long as the pattern of an object against a plain background 

 is visible, the object must be visible. It follows that visibility 

 will to some extent depend upon the blending distance of the 

 pattern. 



The following factors were found to affect the blending distance 

 of patterns : lighting, contrast in tone between the components 

 of the pattern, size of the components and shape of the com- 

 ponents, and the relative size of the components. 



1. Lighting. — Experiments showed that the better the illu- 

 mination the greater the distance at which the pattern was 

 visible. A pattern which by day appears conspicuous, on account 

 of the long distance at which it can be seen, at night may be 

 difficult to see {e. g., the Zebra). 



2. Contrast in Tone between the Components. — The greater the 

 contrast, the greater the blending distance of the pattern. A 

 chequered pattern of black and white is visible at a greater 

 distance than one composed of two shades of grey. 



3. Size of Components. — The larger the components the greater 

 tl>e blending distance, all other factors remaining constant, as was 

 shown in my previous paper (loc. cit.). If there be components 

 of more than one size, then the smaller will blend first and the 

 larger at a greater distance. 



