J. RHEINBERG ON BLACK AND WHITE DOT PHENOMENON. 305 



perhaps not quite apt to call the few bands arising from a single 

 perforation, and which, as mentioned earlier, suffice for the 

 production of the chief image and one above or below, a " trellis- 

 work." 



But apart from these secondary considerations, I do not think 

 the non-shifting of the position of the dots disproves the theory, 

 and for this reason : 



The black dot is invariably surrounded by a space brighter than 



Figr. 2. 



'^O 



the clear part of the diatom, whilst the white dot is invariably 

 surrounded by a darker space than the clear part of the diatom.* 

 The photograph and enlargements which I am passing round will 

 show you this. So that in changing from the black to the 

 w^iite dot focus, the darkness formings the dot does actually shift 

 sideways, and now appears as darkness between two dots. 



This lateral shifting of the darkness is readily observable, and 

 gives rise to the optical illusion that the dots themselves shift in 

 position. 



* By the clear part of the diatom is meant a part of the silex comparatively 

 free from perforations, which, as Mr. Nelson states, approximates to the 

 brightness of the field. 



