112 HUME iv 



sketches, rather than portraits, of the originals 

 the salient features are obvious, while the sub- 

 ordinate characters are obscure or unrepresented. 



Now, when several complex impressions which 

 are more or less different from one another let 

 us say that out of ten impressions in each, six are 

 the same in all, and four are different from all 

 the rest are successively presented to the mind, 

 it is easy to see what must be the nature of the 

 result. The repetition of the six similar impres- 

 sions will strengthen the six corresponding 

 elements of the complex idea, which will there- 

 fore acquire greater vividness; while the four dif- 

 fering impressions of each will not only acquire 

 no greater strength than they had at first, but, in 

 accordance with the law of association, they will 

 all tend to appear at once, and will thus neutralise 

 one another. 



This mental operation may be rendered com- 

 prehensible by considering what takes place in 

 the formation of compound photographs when 

 the images of the faces of six sitters, for 

 example, are each received on the same photo- 

 graphic plate, for a sixth of the time requisite 

 to take one portrait. The final result is that all 

 those points in which the six faces agree are 

 brought out strongly, while all those in which they 

 differ are left vague; and thus what may be 

 a generic portrait of the six, in contradis- 

 tinction to a specific portrait of any one, is produced. 



