32 



On Visual Diffiisivity. 



Some o£ the interesting experiments described by Shelf or d 

 Bidwell * would, with very slight modifications, have shown 

 all the effects which form the subject-matter of this paper. 

 One of his experiments, for instance, was the rotation of a 

 spectrum about an axis beyond the red end. The different 

 apparent positions of the various colours probab ly escaped him 

 owing to their gradual transition one into another, while a 

 crude artificial spectrum of several patches of colour would 

 have revealed the peculiarity. The curved after-image which 

 he shows following the spectrum I believe has that shape due 

 to imperfect fixation, as illustrated in fig. 8, or perhaps to 

 its extending over a sufficient extent of the retina to show a 

 probable variation of diffusivity from centre to periphery. 



One of Bidwell's diagrams, illustrating the fluctuating 

 intensity of the extended image of a rotating bright slit 



Fig. 9. 



Hjhf off 



r^ 



a. Progress of sensation with time due to a '1/5 second exposure 

 according to Bidwell : b, electrical transmission of brief stimulus 

 by line containing- inductance and capacity. 



and its after-image, deserves attention in any discussion of 

 the visual processes as in part phenomena of physical trans- 

 mission. This diagram, produced in fig. 9 a, is to be 

 compared with fig. 9 6, which is the record of a brief 

 * Bidwell, Proc. Roy. Soc. hi. 1894, p. 132. 



