168 Mr. Francis Gallon [April 25, 



is somewliat lighter even than card No. 4, which contains eight 

 portions. 



The same relation is true as regards sound. The difference of 

 noise made by the fall of one shilling or of two shillings is not 

 readily perceived, unless we are specially attending to it. Neither is 

 the difference readily perceived between firing a 38-ton gun or two 

 such guns from the turret of an ironclad, as was proved by the 

 evidence in the late terrible accident on board the ' Thunderer.' 

 Here is an apparatus of eight arms that may be lifted in succession 

 and then let drop by turning a cylinder like that of a musical snuff- 

 box. Each arm as it falls makes the same amount of noise. The 

 catches are so arranged on the cylinder that the effect of turning it is 

 to lift and let drop first one arm, then two arms simultaneously, then 

 four, then all the eight. It will be observed that the apparent loud- 

 ness of sound increases by equal intervals, and not at all as the 

 numbers 1, 2, 4^ 8. 



Finally, two large revolving discs are exhibited under illumina- 

 tion. They are painted black and white in five concentric rings, with 

 a perfectly black centre. In the first of the two discs, counting pure 

 white as 5, the proportions of white to black in the successive rings 

 are as 1, 2, 3, 4, 5, thus forming an arithmetical series. On turning 

 the wheel, the eye utterly repudiates the effect as being that of a 

 series of equally gradated tints, and yet the actual quantities of white 

 form such a series. In the second of the two discs, the proportions of 

 white to black in the successive rings follow Weber's law, or rather, 

 Delboeuf's modification of it ; the disc is, indeed, a reproduction of 

 that described in Delboeuf's memoir. On revolving it, the eye at once 

 recognizes the effect of a beautifully exact gradation ; but in order to 

 show this properly, the illumination has to be very carefully adjusted. 



These illustrations of Weber's law are submitted in order to make 

 manifest the great difference between the progressive increase of 

 objective causes and that of their corresponding subjective effects, 

 and to afford a prima facie evidence of the small influence likely to 

 be exercised upon a generic mental image by a repetition of similar 

 impressions. I do not venture as yet to assert that the law of Weber 

 applies to this case, but the probability of its doing so is pointed out, 

 and also the fact that the true law, whatever it may be, is certainly 

 in some sense analogous to tliat of Weber. According to that law, if 

 it required a tenfold experience or a tenfold period of exposure to 

 produce a mental impression that should contribute to the composition 

 of a blended image in twice as large a degree as a single experience 

 or a single period of exposure, it would require a hundredfold 

 experience or exposure to result in a threefold contribution. 



The law of Weber has a further application to the topics under 

 consideration. When the comparison was made a short time back 

 between the blended image in the artist's brain and the photographic 

 composite, it was stated that a fiftyfold period of exposure would 

 produce in the latter case a fiftyfold effect, in the sense of being 



