KELATIONSHIP BETWEEN STIMULUS AND SENSATION 561 



rays cannot produce photochemical change and cannot be perceived 

 by the eye. It may be mentioned in this connection that few organic 

 pigments a'bsorb strongly in the infra -red. 



The limit at the violet end is less easy to determine because the eye media, 

 in common with a large number of other bodies, have the property of fluoies- 

 cing when the ultra-violet rays fall on them, i.e. they convert them 

 into rays of longer wavelength and therefore make them visible. The 

 resulting impression is however quite different because, since these rays 

 are generated in the eye media themselves, they are spread over the re- 

 tina as a haze without there being any proper image formation. The limit of 

 the visibility of the violet end of the spectrum appears to be at about 3800 

 A.U., while the portion which is seen because of the fluorescence which it 

 produces, and which appears a pale lavender, ends at about 3200 A.U. Since 

 the wavelength of the extreme red rays is a little more than double that of 

 the extreme violet, the eye is sensitive to a little over an octave. The range 

 of appreciation of the eye is therefore very much smaller than that of the ear, 

 which is about 10 octaves. As age increases the eye media become 

 yellow in colour: this change particularly affecting the lens, the violet 

 end of the spectrum becomes shortened owing to absorption. On 

 removing the lens of the eye as in an operation for cataract, 

 the sensitiveness to the violet end of the spectrum is considerably 

 increased. It would therefore seem certain that the limitation of the 

 spectrum of the violet end is largely due to absorption by the eye media and 

 not to inappreciation on the part of the retina. The causes of the limitation 

 of the two ends of the spectrum are therefore different. 



DIFFERENCE THRESHOLDS 



Beside the thresholds for light, colour, time, and wavelength, which 

 we have considered above and which may be called absolute thresholds, 

 there are certain difference thresholds that must be considered. Thus, for 

 example, a certain finite difference must exist between the intensities of two 

 sources of light of the same colour for a difference between them to be appreci- 

 ated by the eye. There are four principal types of difference threshold, that 

 of intensity, that of colour, that of saturation, and that of size. 



Difference Threshold of Intensity 



It is found by experiment that a just perceptible difference between the 

 intensities of two surfaces varies with the mean value of their intensities. 

 Thus supposing that it had been found by one experiment that a difference 

 of intensity of one foot candle was necessary in order that two sources should 

 be just distinguishable, the average intensities of which were one hundred 

 foot candles, then in another case in which the average was 25 F.C. the least 

 perceptible difference would be found to be one-quarter F.C. This condition 

 is known as Weber's law. It appears to be true for light of medium intensity 

 and for sources not separated by more than a small interval. But the least 

 perceptible difference is found by most observers to be less^than that taken 

 for purposes of illustration above, namely one-hundredth- part of the mean 



" 36 



