SECTION VIII 



THE RELATIONSHIP BETWEEN STIMULUS AND 



SENSATION 



IN the introduction I have pointed out that, since we are unable to express 

 our sensations in terms of physical units (we cannot say, for example, when 

 one source is twice as bright as another), two methods of investigation are 

 alone available, namely, that which involves the determination of threshold 

 values, and that which depends on the making of comparisons. 



In order that a source of light shall be perceived, the image which is 

 formed on the retina must have certain properties. In the first place it 

 must last for a certain finite time, for if it be of shorter duration than this 

 it will not be perceived. Secondly it must be larger than a certain 

 size. Thirdly its intensity must be greater than a certain limiting 

 quantity. Fourthly the rays which it emits must have wavelengths 

 which lie between certain limits: So that in the case of each of 

 these four properties there are limiting values which must be 

 exceeded ; these values are called thresholds. The retina has 

 distributed over its surface two different types of sensitive organ, the 

 cone apparatus, which has the function of perceiving colours and is 

 used in day vision, and the rod- visual purple apparatus, which is colour- 

 blind but very sensitive to light of low intensity and is therefore used for 

 twilight vision. Owing to the fact that the distribution of these organs 

 is not uniform, we have to state the part of the retina which is being stimu- 

 lated when assigning a value to any of the above-mentioned thresholds. For 

 example, the threshold for intensity may be that which just actuates the 

 rods, that is the achromatic limit, or that which is sufficient to affect 

 the cones and therefore causes an appreciation of colour. Moreover 

 the value of any one threshold is to a considerable extent controlled 

 by the value of the other factors 'which I have mentioned ; for example, 

 the time threshold is shorter the greater the size and intensity of the light 

 source. The exact conditions must therefore be carefully stated in quoting 

 the value of a threshold. Lastly we must consider the personal equation 

 of the observer, and also the state of his vision, for both are affected by 

 constitution, health, fatigue, &c., to an important extent. 



INTENSITY THRESHOLD FOR LIGHT (ACHROMATIC) 



If a spectrum be gradually reduced in intensity, it loses its colour and 

 finally appears to the eye as a bright band which has its greatest lumi- 

 nosity in the green region of the spectrum and gradually fades towards both 



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