542 THE POPULAR SCIENCE MONTHLY. 



generic image in his brain be identical with the photographic compos- 

 ite ? (I am assuming, for argument's sake, that the photograph gives 

 a true rendering of any optical image, which, in strictness, it does not.) 

 Suppose a succession of many different pictures are to be displayed, 

 each for the same brief period, and if a single other picture is dis- 

 played fifty times in succession, or for fifty times as long, would its 

 share in the generic image be fifty times as large as that of any of the 

 others, or, if not, what would its share be ? 



The reply is, that both in the photographic composite and in the 

 processes of numerical statistics, its effect would be exactly fifty times 

 as great, but in mental imagery this would certainly not be the case, 

 and therein lies a fertile source of error in our general impressions. 

 I have made some experiments on the subject, which are not as yet 

 sufficiently advanced to be worth recording, but I may say that at 

 present I see nothing in the results imcompatible with the very rea- 

 sonable supposition that the relation between the varying periods of 

 exposure and the strength of the corresponding mental impression fol- 

 lows the law of "Weber. This law is founded in the fact that, the more 

 highly our senses are stimulated, the more is their discriminative power 

 blunted. Thus a double number of candles does not double the appar- 

 ent illumination ; it only increases it by a certain amount, which is 

 always the same, whether the light of a single candle be added to that 

 of another single candle, or the light of a thousand candles be added 

 to that of another thousand candles. The law is true of all the senses. 

 The difference of noise made by dropping one shilling or two shillings 

 on a table is not always distinguished by the ear, neither is that of 

 discharging one or two thirty-eight-ton guns from the turret of the 

 same iron-clad ship, as was shown in evidence concerning the recent 

 frightful accident on board the Thunderer. That is to say, the same 

 increment of noise may be produced by the fall of a shilling on a ta- 

 ble, in the one case, as by a thirty-eight-ton gun in the other. 



Let me take the present opportunity of saying that one effect of 

 "Weber's law is that a true composite never appears true, and is never 

 what our uncorrected senses teach us to expect. If we mix a very 

 dark gray with a very light gray, we might on first thoughts expect 

 that their mixture would appear to be a medium gray, but "Weber's law 

 tells us that the eye judges differently, and we find, in trying the ex- 

 periment, that the mixture is brighter than we had expected.* Of 



* Weber's law may be well illustrated by placing in a row, say, five cards, painted 

 quite black, each the size of half a sheet of note-paper. Then taking a whole sheet of 

 white note-paper, tear it in half and lay one half on card 5 so as to cover it entirely. 

 Tear the remaining half exactly across its middle and lay one half upon card 4 ; again tear 

 the remainder in half and lay one half on card 3. Proceed similarly up to card 1 ; the 

 fragment that remains is not wanted. Cut these papers into shreds (excepting No. 5, 

 which can be left as it is), and distribute the shreds as evenly as possible over their 

 respective cards. Then 1 will have one portion of white, 2 will have two portions, 3 will 

 have four portions, 4 will have eight, and 5, which is wholly covered with white, will have 



