THE RETINA. 167 



Experiment 65. Take a thin strip of paper and blacken about 

 1 mm. of its end. Hold a large sheet of paper with a dot in 

 the centre vertically in front of you. Keeping your head quite 

 still, close one eye and look fixedly at the dot with the other. 

 Move the blackened end of the paper strip slowly over the field 

 of vision and mark the points when it seems to disappear. 

 From these points map out the area of the blind spot in the 

 visual field. Using the schematic eye, show where it must be 

 in the retina. 



The Response of the Retina to Light. In addition to the 

 information from subjective experiments such as those which we 

 are about to discuss, which of course involve activity of the brain 

 as well as the eye, some interesting knowledge as to the nature of 

 the retina's reaction to light stimuli has been got from direct 

 experiments on the eye itself. The technique which shows the 

 movement of the cones, the negative variation in the retina, and 

 the formation and destruction of the visual purple, is unfortunately 

 too difficult for class experiments. The student must turn to his 

 text-books for information as to the nature and significance of this 

 work. 



DARK ADAPTATION. It is familiar that eyes at night or in a dark room can see 

 lights so faint that they would not be made out at all by day. This is in part due 

 to an actual lowering of the threshold of the light-sensitive elements, and one may 

 satisfy oneself by simple experiments that the main part at least of this change 

 in threshold occurs, not in the line of direct vision, but in the peripheral field. 

 The observations are entirely subjective ones and the student must make them 

 for himself under suitable conditions of lighting. If one looks at the sky at twi- 

 light searching for the first star it may appear on one side or another of the direct 

 line of vision, only to disappear when looked at directly. Small pieces of white 

 paper on a black background in a darkened room are visible as long as they are 

 not in the direct line of vision, but disappear, each as its image is made to fall on 

 the fovea. Indeed, workers in this field have not been able to find that any 

 lowering of the threshold at all occurs in the fovea during adaptation to the dark 

 and this makes it probable that the change in the threshold of the peripheral 

 field is a change in the rods and is not shared by the cones. The adaptation 

 which the rods undergo is thought to be associated with the presence in them 

 of visual purple, the coloured substance which is formed in them in the dark. 

 One of the chief reasons for this is the striking resemblance between the sen- 

 sitiveness of the pigment on one hand, and of the dark-adapted eye on the 

 other, to waves of different lengths. When several different colours are seen 

 under similar lighting conditions it is only a matter of judgment which of them is 



