THE SUBJECTIVE PHENOMENA OF VISION 567 



Similar oscillations occur when the telegraph circuit is broken ; these 

 oscillations also have been witnessed and described by Bidwell at the 

 end of the primary visual response. The time taken for the sensa- 

 tion to reach its maximum varies from 0'16 to 0*07 sees., being shorter 

 the greater the intensity of the stimulus. This so called primary 

 image is followed under certain conditions by a less definite and less 

 intense image, which has the following characteristics : (1) It is not 

 seen when the eye is light adapted. (2) It is strongest for green light, and 

 is absent for red. (3) It is absent from the fovea. (4) It is not seen by 

 persons suffering from night blindness. (5) It is always of a bluish grey 

 colour. All the above facts fit in with the view that, whereas the primary 

 response corresponds with the reaction of the cones to the stimulus, the 

 image which is sometimes seen to follow belongs to the rod apparatus. 

 THE AFTER IMAGE. Following these responses of the cone and rod 

 end-organs is the so-called secondary image, which certainly concerns the 

 cones and may concern the rods as well. This image is of longer duration 

 than those already considered, and it is of much lower intensity. It has 

 however the peculiarity that, so long as it lasts, the part of the retina affected 

 gives a diminished response to a stimulus of the same type as that which 

 it had previously received. For exampe, if the first stimulus was one of 

 white light, then a second white light stimulus falling in the period occupied 

 by the after image of the first, would not be recorded by the retina with the 

 normal intensity. It has been pointed out that the after image period in 

 some ways resembles the refractory period which follows the activity of 

 muscle and nerve. If the second stimulus occupies a larger area of the 

 retina than that on which the first stimulus fell, the first area stimulated 

 appears dark on the bright ground corresponding to the second area. If 

 the first stimulus is coloured, and no. stimulus follows, the secondary 

 image is found to have the same colour ; but if a second stimulus of the same 

 colour falls on the retina during the secondary image, then as before the 

 area first stimulated appears dark on the bright area occupied by the second 

 area. If on the other hand the second stimulus be one of white light, 

 the sensation received is one of the complementary colour, the reason being 

 that the red constituents of the white light are partially excluded 

 by the after image of the .first stimulus, but not so the other spectral 

 colours, and the image which is seen is therefore a blue-green one. Because 

 of these peculiar properties the after image is said to have two phases : being 

 called positive when the eye receives no second stimulus, and the appearance 

 of the after image is the same as that of the first stimulus ; and being called 

 negative when, owing to the incidence of a second stimulus, the after image 

 shows the opposite intensity or colour to the stimulus which originated it. 

 Since absence of the second stimulus causes the after image to be positive 

 and the presence of a second stimulus makes it appear negative, we should 

 expect a second stimulus of the right intensity to cause the after image to 

 disappear altogether, since it would stimulate the surrounding retina with 

 the same intensity as does the after image of the first stimulus. Experiment 



