LXXII.] 



DIRECT VISION. 



343 



The first effect is white, followed hy an after-effect which is black 

 even during the continued white stimulus. Thus there seems to be a 

 slow oscillatory process in the retino-cerebral apparatus, showing a 

 positive and a negative phase, each phase lasting 0.014" to 0.016". 



The negative phase of oscillation takes place after the shortest 

 possible illumination, and appears to be a general phenomenon. 

 Charpentier suggests that it is possible that a single bright stimulus ; 

 e.g , an electric spark, appears as a double or reduplicated bright 

 sensation (Archives de Physiologie, 1892, p. 541). Another form 

 of the experiment is given in a later paper (p. 629). 



FIG. 260. Charpen tier's Disc for 

 "Black Band." The arrow 

 shows the direction of rotation. 



FIG. 261. Charpentier's Disc for Vision 

 of Purple Background. 



(ii.) On a large black disc (40 cm. diameter) gum near its circumference a 

 piece of white paper (i cm. and angular deviation i-2), and cause the disc 

 to revolve twice per second. The observer has a sensation of a white ribbed 

 streak (about \ of the entire circle) on the black surface. There is not a 

 uniform tint, and the ribbed appearance is due to an oscillatory process in 

 the retino-cerebral apparatus. 



(iii.) Arrange a black disc with narrow open equidistant sectors, to rotate 

 opposite to a white surface illuminated by direct sunlight. The sectors have 

 their apices towards the periphery and their bases at the centre (fig. 261). On 

 rotating the disc before the eyes so that the retina is stimulated 40-60 times 

 per second, i.e., when each stimulus oocurs during the negative after-effect 

 of the preceding stimulus, one gets a sensation of a purple-violet field, but the 

 field is colourless at lower or higher rates of stimulation. Charpentier thinks 

 that the coloured sensation is due to entoptical vision of the retinal purple. 



