352 



Mr. T. 0. Porter. 



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disc from practically the same stand-point and distance, the latter 

 being that of most distinct vision ; moreover, he kept his eyes fixed 

 on the same part of the disc, so that the image of the disc did not 

 travel over the retina. AJ1 these are important details as will appear 

 later. 



Before examining the curves in fig. 3, consider a disc rotating under 

 a fixed degree of illumination, i.e., in a light of constant intensity. 

 Suppose that the light is of a certain colour, e.g., yellow, and that 



