460 



Mr. C. Lloyd Morgan. 



But the brightness or sensation-luminosity is certainly far removed 

 from the arithmetical mean between that due to white and that pro- 

 duced by black. The fact is, perhaps, even more clearly brought out 

 if we divide a disc into eleven concentric areas of equal width, of 

 which the inner is all white and the outer all black, while the inter- 

 vening areas have sectors giving a series of 10 per cent, increments of 

 white. On setting such a disc in rotation a series of concentric grey 

 rings is obtained. Now if the equal increments of stimulus produced 

 equal increments of sensation, the ten steps leading from black to 

 white should appear to be of equal value. But they appear to be of 

 very unequal values. While the step from black to the darkest grey 

 involves a large stride in sensation, seemingly almost half-way towards 

 the white, that from white to the lightest grey is of no great amount. 

 Nor is this difference materially altered by reversing the order of the 

 rings. With steps proceeding from inner black to outer white their 

 inequality for sensation is just as obvious. 



No doubt in reaching this conclusion we are dependent on the 

 exercise of comparison and judgment. We must compare the value of 

 the steps from ring to ring in order that we may perceive their 

 inequality. But the inequality is not a property of the perception but 

 of the visual sensations which are perceived to be separated by unequal 

 intervals. We cannot investigate sensations at all without passing 

 judgment upon them. It is fatal, however, to clear thinking to confuse 

 the act of judgment with the sensory data on which such judgment is 

 passed. 



It is noteworthy that the rings afforded by such a disc when in 

 rapid rotation are not uniform in shade. Apart from the differences 

 of luminosity for sensation between ring and ring, the shade of grey 

 within any selected ring is not the same throughout its width. There 

 is the same percentage of white stimulus throughout its breadth ; but 

 there is not the same brightness for the eye between its limiting 

 boundaries. When the ring adjoins its lighter neighbour it appears 

 distinctly darker than it does on that side which is in juxtaposition to 

 its darker neighbour. This is unquestionably due to the effects of 

 contrast, through the subjective influence of which each ring is differ- 

 entiated in sensation, though there is no corresponding differentiation 

 in the exciting stimulus. It is noteworthy, too, that this contrast 

 effect is more marked in the darker rings than it is in the lighter rings. 

 We have here a disturbing element, for which we must be prepared to 

 make the necessary allowance. For the present, however, we may 

 assume that, though introducing a factor which somewhat distracts 

 the judgment, the disturbance is not sufficient to invalidate the con- 

 clusion that equal, or approximately equal, increments of stimulus 

 produce increments of brightness which differ widely in value. 



We may next endeavour to ascertain whether we cannot by experi- 



