1879.) on Oeneric Images. 169 



equivalent to fifty layers of transparent colour. It was not intended 

 to imjily by this that the tint as estimated hy the eye would be fifty 

 times increased in depth. The law of Weber tells us that it would not 

 be anything like so deej^ as that in appearance. Objectively speaking 

 the tints of a photographic composite are correct, but subjectively speak- 

 ing they are not. Hence there are three degrees of accuracy, respec- 

 tively corresponding to the three processes of (1) numerical averages, 

 (2) of optical or photographic composites, and (3) of mental images. 

 Numerical averages are absolutely correct in every sense. Optical 

 and photographic composites are objectively correct, but subjectively 

 incorrect. Mental images are objectively incorrect, and they are 

 subjectively incorrect in a double degree. Supposing Weber's law 

 to be applicable throughout, a white mark in any one of the portraits 

 would leave a mark on the optical or photographic composite whoso 

 apparent intensity would vary as the logarithm of the time of photo- 

 graphic exposure, but the intensity of the white mark that it would 

 leave on the mental composite would be only as the logarithm of that 

 logarithm. 



Even this result is much too leniently calculated. It is based on the 

 supposition that the visualising power is perfect, the memory absolutely 

 retentive, and the attention perfectly free from bias. This is very far 

 from being the case. Again, some of the images in every presumed 

 generic group are sure to be aliens to the genus and to have become 

 associated to the rest by superficial and fallacious resemblances, such 

 as common minds are especially attentive to. Seeing, as we easily 

 may, what monstrous composites result from ill-sorted combina- 

 tions of portraits, and how much nicety of adjustment is required to 

 produce the truest possible generic image, we cannot wonder at the 

 absurd and frequent fallacies in our mental conceptions and general 

 impressions. 



Our mental generic composites are rarely defined ; they have that 

 blur in excess which photographic composites have in a small degree, 

 and their background is crowded with faint and incongruous imagery. 

 The exceptional effects are not overmastered, as they are in the photo- 

 graphic composites, by the large bulk of ordinary effects. Hence, in 

 our general impressions far too great weight is attached to what is 

 strange and marvellous, and experience shows that the minds of 

 children, savages, and uneducated persons have always had that 

 tendency. Experience warns us against it, and the scientific man 

 takes care to base his conclusions upon actual numbers. 



The human mind is therefore a most imperfect apparatus for the 

 elaboration of general ideas. Compared with those of brutes its 

 powers are marvellous, but for all that they fall vastly short of per- 

 fection. The criterion of a perfect mind would lie in its capacity of 

 always creating images of a truly generic kind, deduced from the 

 whole range of its past experiences. 



General impressions are never to be trusted. Unfortunately when 

 they are of long standing they become fixed rules of life, and assume 



Vol. IX. (No. 71.) n 



