LXXII.] 



DIRECT VISION. 



3*43 



The first effect is white, followed by an after-eftect 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 Phtjsiologie, 1892, p. 541). Another form 

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



FlO. 260. — Charpentier*s Disc for 

 "Black Band." The arrow 

 shows the direction of rotation. 



FlO. 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 diso 

 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 a])paratus. 



(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 thf;ir bases it the centre (fig. 261 ), On 

 rotating tho 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. 



