SOME CURIOSITIES OF VISION. 205 



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

 7, the red color being represented by shading. 



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

 effect, 1 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 the 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 exper- 

 imentally : 



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 disk of nearly uni- 

 form luminosity. When this was moved rather quickly, either back- 

 ward or forward, or round and round 

 in a small circle, the edges of the 

 streaks of light thus formed appeared 

 to be bordered with red. 



If this experiment is performed with 

 a strong light, the hole becomes bor- 

 dered with greenish blue instead of 

 red. With an intermediate degree of 

 illumination both blue and red may 

 be seen together, the blue being 

 inside the red. 



Most of the effects that have so far 

 been described were produced by transmitted light, but reflected light 

 will show them equally well. If you place a printed book before you 

 near a good lamp and interpose a dark screen before your eyes, then, 

 when the screen is suddenly withdrawn, the printed letters will for a 

 moment appear red, quickly changing to black. Some practice is 

 required before this observation can be made satisfactorily, but by a 

 simple device it is possible to obliterate the image of the letters before 

 the redness has had time to disappear ; the color then becomes quite 

 easily perceptible. Hold two screens together side by side, a black 

 one and a white one, in such a manner that there is a triangular open- 

 ing left between them. In the first place let the black screen cover the 

 printing, then quickly move the screens sideways so that the printed 

 letters may be for a moment exposed to view through the gap, stop- 

 ping the movement as soon as the page is covered by the white screen. 

 During the brief glimpse that will be had of the black letters while 

 they are beneath the gap, they will, if the illumination is suitable, 

 appear to be bright red. 



We may go a step further. Out out a disk of white cardboard, divide 



1 See Nature, Vol. LV, p. 367 (February 18, 1897;. 



