VISUAL SENSATIONS. 767 



illuminated by several rapid flashes in succession, several distinct images 

 corresponding to the positions of the body during the several flashes are 

 generated ; the images of the body corresponding to the several flashes fall 

 on different parts of the retina. 



658. The intensity of the sensation varies with the luminous intensity 

 of the object ; a wax candle appears brighter than a rushlight. The ratio, 

 however, of the sensation to the stimulus is not a simple one. If the lumi- 

 nosity of an object be gradually increased from a very feeble stage to a very 

 bright one, it will be found that though the corresponding sensations like- 

 wise gradually increase, the increments of the sensations due to increments 

 of the luminosity gradually diminish ; and at last an increase of the lumi- 

 nosity produces no appreciable increase of sensation ; a light, when it reaches 

 a certain brightness, appears so bright that we cannot tell when it becomes 

 any brighter. Hence it is much easier to distinguish a slight difference of 

 brightness between two feeble lights than the same difference between two 

 bright lights ; we can easily tell the difference between a rushlight and 

 a wax candle ; but two suns, or even two bright lamps, one of which differed 

 from the other merely by just the number of luminous rays which a wax 

 candle emits in addition to those sent forth by a rushlight, would appear to 

 us to have exactly the same brightness. In a darkened room an object 

 placed before a candle will throw what we consider a deep shadow on a 

 sheet of paper or any white surface. If, however, sunlight be allowed to 

 fall on the paper at the same time from the opposite side the shadow is no 

 longer visible. The difference between the total light reflected from that 

 part of the paper where the shadow was, and which is illuminated by the 

 sun alone, and that reflected from the rest of the paper which is illuminated 

 by the candle as well as by the sun, remains the same ; yet we can no longer 

 appreciate that difference. 



On the other hand, if using two rushlights we throw two shadows on a 

 white surface and move one rushlight away until the shadow caused by it 

 ceases to be visible, and, having noted the distance to which it had to be 

 moved, repeat the same experiment with two wax candles, we shall find 

 that the wax candle has to be moved just as far as the rushlight. In fact, 

 it is found by careful observation that, within tolerably wide limits, the 

 smallest difference of light which we can appreciate by visual sensations is 

 a constant fraction (about T -^o tn ) f tne tota l luminosity employed. The 

 same law holds good with regard to the other senses as well. The smallest 

 difference in length we can detect between two lines, one an inch long and 

 the other a little less than an inch, is the same fraction of an inch that the 

 smallest difference in length we can detect between a line a foot long and 

 one a little less than a foot, is of a foot. Put in a more general form, then, 

 the law, which is often called Weber's law, 1 is as follows : When a stimulus 

 is continually increased, the increase of stimulus necessary to call forth the 

 smallest appreciable increase of sensation always bears the same proportion 

 to the whole stimulus. 



659. Distinction and fusion of sensations. When light falls on a large 

 portion of the retina the total sensation produced is greater in amount than 

 when a small portion only of the retina is affected ; a large piece of white 

 paper produces a greater total effect on our consciousness than a small one, 

 though, if the surfaces be uniformly and equally illuminated, the intensity of 

 the sensation is in each case the same ; the small piece of paper appears as 

 bright or as " white" as the large one. If the images of two luminous ob- 

 jects fall on the retina at sufficient distances apart, the consequent sensations 



1 From which Fechner, by an assumption, obtained a mathematical expression or for- 

 mula, which is sometimes incorrectly spoken of as Fechner's law. 



