362 Mr. Shelf ord Bidwell [March 5, 



-^ inch or 1 mm., and the appearance lasts for something like 

 ytj. second. Most people are at first quite unable to recognise this 

 effect, the difficulty being not to see it but to know that one sees it. 

 Those who have been accustomed to visual observations generally 

 perceive it without any difficulty when they know what to look for, 

 and no doubt it would be quite evident to a baby a few weeks old, 

 which had not advanced very far in the education of its eyes. 



The observation is made rather less difficult by a further device. 

 If the disc^ is divided into two parts by an opaque strip across the 

 middle, it is clear that each half-disc will have its red border, and, if 

 the strip is made sufficiently narrow, the red borders along its edges 

 will meet, or perhaps overlap, and the whole strip will, for a moment 

 after the shutter is opened, appear red. A disc was prepared by 

 gumming across the paper a strip of tin foil about 3L inch wide. 

 The effect produced when such a disc is exposed is indicated in 

 Fig. 7, the red colour being represented by shading. 



Fig. 7. 



A simpler apparatus is, however, quite sufficient for showing the 

 effect,* and with practice one can even acquire the power of seeing it 

 without any artificial aid at all. I have many times noticed flashes of 

 red upon the black letters of a book that I was reading, or upon the 

 edges of a page : bright metallic or polished objects often show it 

 when they pass across the field of vision in consequence of a move- 

 ment of the eyes, and it was an accidental observation of this kind 

 which suggested the following easy way of exhibiting the effect 

 experimentally. 



An electric lamp was fixed behind a round hole in a sheet of 

 metal which was attached to a board. The hole was covered with 

 two or three thicknesses of writing paper, making a bright disc of 

 nearly uniform luminosity. When this was moved rather quickly 

 either backwards or forwards or round and roimd in a small circle, 



* See ' Nature,' vol. Iv. p. 367 (Feb. 18, 1897). 



