464 



Mr. C. Lloyd Morgan. 



faulty observation. This can now be readily put to the test of further 

 experiment. A ring disc can be constructed on the basis of the new 

 curve. But this on rotation affords steps which are of very distinctly 

 unequal value to the eye. 



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There is therefore a real discrepancy for sensation between the 

 results obtained by the method of continuous shading, and those 

 obtained by the method of graded steps. May it not be due to those 

 effects of contrast to which attention has already been drawn 1 To 

 test the validity of this suggestion attempts were made to get rid of 

 the effects of contrast within each ring, and in doing so, to obtain a 

 rough quantitative measure of these effects. We have seen that each 

 ring appears too light on that side which adjoins a darker neighbour, 

 too dark on the other border where it is in contact with a lighter 

 neighbour. Either by increasing the amount of white stimulus on its 

 darker side, or by decreasing that amount on its lighter side, the ring 

 may be made to appear of uniform shade throughout. It was found 

 that approximately the same proportional amount of white must be 

 added at one border or subtracted at the other border to produce this 

 result. 



Taking the step disc, which gives fairly equal sensation increments, 



