684 



DR. J. C. MOTTRAM ON 



within blending distance, and against backgrounds closely similar 

 to this background the checkered disc is at a disadvantage as 

 regards invisibility. Over a long series of experiments there was 

 some indication that against other backgrounds the checkei'ed disc 

 was slightly less visible, the measure being the greatest distance 

 at which the discs could be seen ; but although great care was 

 taken with the constants of the experiment, the differences were 

 too small and the experimental error too large, to justify a definite 

 conclusion, particularly as the appearance of the checkered disc 

 after blending was remarkably difierent from that of the plain 

 disc. When the plain disc was visible, its outline was seen to be 

 sharp, so tliat its shape could easily be recognised, but with the 



Text-fiofure 3. 



^ 'i I 5 



7 





'"9 O 



O O . 



DO 10. 



// :l 



Examples of obliterative shading by means of pattern-blending 



checkered disc the outline was remarkably indistinct, so that 

 it was frequently impossible to tell whether a square, circle 

 or equilatei'al triangle had been placed against the background. 

 Experiments were carried out in order to determine the factors 

 controlling this obscuring of form. Four squares were prepared 

 as shown in text-fig. 3, 2-5 : as can be seen, these squares consist 

 of black and white areas of equal proportions. In fig. 3 the areas 

 are interrupted along the two opposite margins, in fig. Jf along 

 the two adjacent margins, in fig. 5 along all four edges, and in 

 fig. 5 there is no iuteri-uption at the margins. These squares . 

 were examined under difierent conditions of liiiiitinii- and against 



