370 PHYSIOLOGY CHAP. 



Positive and negative after-images have already been discussed 

 from the point of view of luminous intensity or brightness, but 

 without regard to their colour-tone. AVe must now consider 

 t-ntourrd after-images. 



Generally speaking, after lixating it for a few seconds, a 

 coloured object yields an after-image which may be positive, i.e. 

 of the same colour, but is more often negative, i.e. of the comple- 

 mentary colour blue-green after red, violet after yellow, blue 

 after orange, and vice rersa. 



If the eyes are first kept for a few moments in complete dark- 

 ness, to increase the sensitiveness of the retina, and to obviate the 

 after-effects of all previous impressions, and are then opened 

 suddenly in daylight and then closed again, a positive after- 

 image of external objects appears. If while the image is still 

 plainly visible the eves are again opened, and directed upon a 

 white surface, the i>*i(ive is transformed into a negative image, 

 and assumes the complementary colour of the object fixated. 



According to rVehner the positive homochro tinifir image (i.e. of 

 the same colour as the light that induces it) seen in the dark is 

 due to the persistence of the chromatic excitation, and the comple- 

 mentary negative image seen on exposure to white light is due to 

 fatigue of the retina to the inducing colour, with unaffected 

 excitability to all other colours of the spectrum in the admixture 

 of which the complementary colour predominates over that to 

 which the retina has been fatigued. Johannes Midler had previ- 

 ously given a similar interpretation of this phenomenon. 



White light, too, when it acts intensely on the retina produces 

 coloured after-images, which alternate from red to green, and 

 appear and disappear at a certain rhythm a phenomenon 

 described under the name of coloured phases of the after-images. 

 They may be regularly observed on looking fixedly for a few 

 seconds, with the dark-adapted eye, at a window on which the sun 

 is shining, or at the bright light of a lamp. This phenomenon has 

 been adequately described by Fechner, Sequin, Plateau, Helniholtz, 

 and others ; it occurs not only with white light, but also with 

 brightly illuminated saturated colours, although their phases are 

 not so definite. 



Various hypotheses have been suggested in explanation. 

 Generally speaking, it may be said that after vigorous stimulation 

 the excitability of the retina to various colours passes through 

 a series of positive and 'negative oscillations; the cerebral visual 

 centres consequently receive alternate impulses which determine 

 complementary colour- sensations and periodic appearance and 

 disappearance of the images. 



It may also be stated that, according to our present knowledge, 

 after-images whether coloured or colourless are associated only 

 with peripheral changes in the activity of the retina, and that no 



