292 THE GROWTH OF THE BRAIN. 



may or may not be recognised. Pierce and Jastrow z 

 have reported observations bearing on just this matter. 

 Briefly these interesting experiments were as follows : 

 Two illuminated surfaces were compared when the in- 

 tensity of the illumination differed by a very slight yet 

 measurable amount, the subject being required to state 

 which surface was the brighter. The difference was so 

 slight that it could not be recognised, and the subject 

 was therefore compelled to " guess." The result of 

 " guessing " showed that the brighter was correctly 

 designated with a frequency so great that the unre- 

 cognised difference was clearly effective in determining 

 the choice. The observations have shown that differ- 

 ences too small to be discriminated may still influence 

 our reactions, and it is thus seen that among effective 

 stimuli there must also be included those which we do 

 not recognise. 



1 Pierce and Jastrow, Mem. Nat. Acad. Sci., Washington, 1884. 



