RELATIONSHIP BETWEEN STIMULUS AND SENSATION 559 



they make at the eye must be greater than a certain limiting value. The 

 angle usually obtained is an angle of one minute, and on this the lettering 

 used in practice for testing the visual acuity of patients is based (see 

 page 535). Persons with exceptionally good vision are able to see 

 the images separated when the angle which the sources make at the 

 eye is very much less than this, namely 24 seconds. Assuming that 

 the posterior nodal point is 15 '6 mm. from the retina (this being the 

 distance in the normal eye, see page 523) 24 seconds corresponds with a 

 distance between the images at the retina of '0018 mm. The diameter 

 of the cones is between '0020 and 'C030 mm., and in the fovea they are 

 very closely packed so as to present a hexagonal section. The maximum 

 visual acui'ty is therefore certainly as great as the size of the cones would 

 lead us to suppose possible. 



The case of a dark spot on a bright ground is similar to the case just 

 considered, because for the dark spot to be recognised it must subtend 

 at the eye the minimal angle mentioned above. Increasing the intensity 

 of the ground or the blackness of the spot will make a very small difference. 

 The case of the black spot is therefore very different to that of the white 

 in which an increase in the intensity is sufficient to make up for a difference 

 of size. 



TIME THRESHOLD 



In considering the time threshold two different sets of conditions require 

 description, firstly the minimum time during which a given stimulus must 

 act in order to reach consciousness, and secondly the minimum rate at which 

 a series of stimuli must follow one another in order to give a uniform impres- 

 sion without flicker. Both are of considerable importance since the first 

 enters such problems as the determination of the length of time during 

 which a lighthouse beam should be caused to travel in a given direction, 

 the second because it gives a reliable method of comparing the 

 intensity of lights of different colour. Experimental investigation of the 

 first type of time threshold is effected by measuring the length of stimulus 

 necessary to cause a source of a certain intensity to affect the retina, and 

 it is found that the lower the intensity the longer must the image fal) on the 

 retina. But if the eye be dark adapted, if the time and the intensity 

 values be multiplied together, then within limits a constant is obtained. 

 On the other hand, in the light adapted eye, the value is found to vary 

 somewhat, but is sufficiently constant to show that the relation between 

 intensity and time is approximately the same. Within limits therefore we 

 find that at the threshold the total amount of light is constant whether it 

 be of high intensity for a very short period of time or of low intensity for 

 a correspondingly longer one. This relationship ceases to be true if the time 

 of stimulation is longer than about one- tenth of a second, and this is appar- 

 ently due to the fact that the retinal apparatus reaches a steady state in 

 about one- quarter of a second in the dark adapted eye (rod- visual purple 

 apparatus). A lighthouse flash should therefore be visible to the eye for this 



