GENERIC IMAGES. 543 



course, we could learn by much practice to correct the judgment of 

 our senses, but it is only in rare and special cases that we have the 

 necessary practice. I have often noticed my own ludicrous failures in 

 estimating the relative depths of two parts of the same pool by the 

 relative obscurity of the bottom. Maps of ocean-depths are never 

 made on what may be called natural scales, but always on symbolic 

 ones, in which consecutive increases of tint, as judged by the eye, cor- 

 respond to successive increases of depth. According to Weber's law 

 (which I content myself here with expressing in its original and ap- 

 proximative form), if it requires a tenfold period of exposure to make 

 a doubly deep impression on the mind, it would require a hundred-fold 

 period to make a trebly deep one, a thousand-fold period to make it 

 quadruply deep, and so on. The one series follows an arithmetical, 

 the other a geometrical progression. 



Whatever the true law may be that connects the strength of the 

 impression with the time that the object is before our eyes, or with 

 the frequency with which it is seen, its form is certainly not very dis- 

 similar to that of the law of Weber. Otherwise it would not accord 

 with the fact that sights on which we have not lingered, often leave 

 abiding impressions, while the pictures that hang on our walls, before 

 our eyes, every day of our life, are not always remembered with vivid 

 distinctness. The effect of the law, whatever its precise form may be, 

 is to prevent generic images from having the same definition and 

 simplicity as the corresponding photographs. The most extreme ele- 

 ments will always leave their traces very visibly because the medium 

 elements are not present in sufficient number to overpower them. 

 These images can not be otherwise than blurred and surrounded by 

 monstrous and faint imagery. The attention is unable to deal with 

 such pictures, because when it is engaged on one part of them the 

 remainder slips out of memory. All parts of an image must be con- 

 gruous and well defined before the attention can sweep so swiftly over 

 the entire field of view as practically to bring it all at once into sight. 

 If an image is incongruous and vague, the mind follows the course 

 already described when the illustration was used of a clergyman in a 

 pulpit. 



The conclusions to be drawn from what I have said are that com- 

 posite portraits are perfectly trustworthy when made by optical means 

 and with proper precautions, and that photographic composites are as 

 correct representations of these as photographs ever are of the pictures 

 from which they are taken. Composite portraits are therefore to be 

 considered as pictorial statistics. Also it is conceivable that general 



sixteen. The effect of the scattered white on the cards is to produce various grays which 

 the eye will judge to be separated by equal intervals of tint. Card 4, which contains 

 eight portions, has the medium amount of white (eight and a half is the precise medium), 

 but the eye reckons differently ; it places the medium tint at card 3, which contains only 

 four portions of white. 



