72 VISUAL SENSATIONS. [BOOK m. 



small that the image of it on the retina may be considered as a 

 mere point ; we may speak of it as a luminous point. If for the 

 sake of illustration or otherwise we have to consider a larger lu- 

 minous object, we shall do so without regard to the size of the 

 image on the retina unless this is specially mentioned. 



We may begin with the preliminary remark that in dealing 

 with light as a stimulus of visual sensations, we have to consider 

 not only the intensity of the stimulus, but also its duration. A 

 luminous point may appear dim and feeble, that is to say, the 

 waves of light from it have a small amplitude and so bring little 

 energy to bear on the retina, or it may appear bright and strong, 

 that is to say, the waves of light have a large amplitude and so 

 bring much energy to bear on the retina. - Whether dim or 

 bright, the luminous point may act on the retina for a longer or 

 a shorter time ; and, moreover, during its action may remain 

 steady, not varying in intensity, or may vary in intensity and 

 become unsteady or nickering. In estimating the total visual 

 effect of a luminous point, we have to consider both these 

 features, its intensity or brightness and its duration. 



Neglecting for the present the feature of duration, we find 

 that a luminous point must possess a certain amount of brightness 

 in order to produce any conscious sensation at all, in order to 

 be visible. If the waves of light fall on the retina with less 

 than a certain amplitude, if their energy sinks below a certain 

 minimum, they fail to give rise to visual impulses, or at least to 

 such as can affect consciousness.; for we may suppose that visual 

 impulses might be generated and yet be so feeble as not to pro- 

 duce in the cerebral centre changes sufficiently great to affect 

 consciousness. It will be understood, of course, that the exact 

 degree of brightness at which the luminous point becomes visible 

 depends on the greater or less irritability, on the sensitiveness, of 

 the retina. The same amount of luminous energy which, falling 

 on one retina or on one part of a retina, produces a distinct 

 sensation, may, falling on a less sensitive retina or on a less 

 sensitive part of the same retina, produce no sensation whatever. 



From the minimum onwards the intensity of the sensation 

 increases with the luminous intensity of the object ; a wax candle 

 appears brighter than a rushlight, and' the sun brighter than any 

 candle ; we are dealing now with the intensity of the light quite 

 apart from the size of the luminous object. The ratio, however, 

 of the sensation to the stimulus is not a simple cue. If the 

 luminosity 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 likewise gradually increase, the incre- 

 ments of the sensations due to increments of the luminosity 

 gradually diminish, and at last an increase of the luminosity 

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

 reaches a certain brightness, appears so bright that if it becomes 



