SENSATIONS OF COLOUR. Ill 



by an instantaneous excitation (say of -rj^") at first increases, 

 culminates about .j." after the excitation and then rapidly 

 diminishes ; (2) that of a succession of such excitations 

 following each other without intermission, all are at first 

 (during the first tenth of a second) equally effective, so 

 that the sensation occasioned by the series is equal to the 

 sum of the sensations which would have been produced by 

 all of the excitations had they occurred separately ; but 

 afterwards the excitations become less and less effective. 

 Hence it results, first, that in the case of illuminations of 

 the unexhausted retina of about T y duration or less, the 

 sensation of light is proportional to the product of the 

 intensity and duration of the illumination ; and secondly, 

 that when the illumination is continued, the sensation of 

 light at first increases, then gradually diminishes. This 

 diminution of the excitability of the retina by previous 

 excitation is called " exhaustion." In consequence of it, 

 when we contemplate a bright object and then look else- 

 where, we see a dark image (called an after-image) of it. 



When the retina is excited by homogeneous rays, i. e., 

 by rays of which all are of the same refrangibility, the 

 effects follow each other in the same order, but it is 

 found that the colour-sensations occasioned by rays of 

 equal intensity, but different refrangibility, culminate at 

 different rates. The exhaustion produced by monochro- 

 matic light affects the excitability of the retina only in 

 respect of light of the same kind. Consequently, the 

 after-images of coloured objects are themselves coloured. 



Colour sensations are said to be " blended " when the 

 rays which occasion them affect the retina, either simul- 

 taneously or in such rapid succession that their action is 

 simultaneous. If two kinds of light act simultaneously 

 on both retinae, they may also give rise to a blended 

 sensation. 



Classification of colour-sensations : The sensations oc- 

 casioned by monochromatic rays of different kinds admit 



