THE SENSE OF VISION 621 



as by placing the chin in a tin mug, and place a sheet of white paper 

 vertically in front of it at a distance of 18 inches. Draw a dot in the 

 centre of the paper. Close one eye, and with the other regard the dot. 

 Take a thin strip of white cardboard, and blacken about 2 millimetres 

 of the end. Move the blackened end over the region of the field of 

 vision corresponding to the blind spot, and note the points where the 

 black area disappears, marking them on the white paper. A sufficient 

 number of these points can be taken, and a curve drawn through them 

 will indicate the margin of the field of the blind spot. 



3. At the fovea centralis there are chiefly cones; the other layers 

 of the retina are thinned out. This is the spot of acute vision. 



FIG. 353. To DEMONSTBATE THE BLIND SPOT. 



When held about 12 inches away, with left eye closed, the line on the left appears 



continuous. 



The Perception of Light, and the Relation of the Sensation to the 

 Stimulus. We have seen that the sensation of light is due to the 

 stimulation of the receptor mechanism of the eye by means of the 

 vibrations of the ether. According to the amplitude of these vibra- 

 tions, so the sensation of light varies in intensity. According to the 

 length of time the waves fall upon the retina, so the sensation varies 

 in duration. The sensation evoked lasts much longer than the 

 stimulus. The sensation of a flash of lightning or of an electric spark, 

 for example, is much longer than the time during which vibrations 

 are actually falling on the retina. When the stimuli fall in sufficiently 

 quick succession, the sensations become fused, so that a lamp swung 

 quickly in a circle gives the sensation of a ring of light. In moving 

 pictures such a fusion of sensation takes place. For such a fusion 

 to take place, it is necessary for the stimuli to be about ten a second 

 for weak light, and forty a second for strong light. The length of 

 interval varies with different colours, being longest with blue, shortest 

 with yellow, and intermediate with red. 



From the consideration of an electric light and a candle it is obvious 

 that the intensity of the sensation varies with the luminous intensity 

 of the object. Following Weber's law, it is found that it is easier to 

 distinguish a slight difference in brightness between two feeble lights 

 than the same difference in brightness between two bright lights. 

 The smallest difference which can be appreciated in the case of the 

 eye moderately stimulated by light is about T ^ of the total luminosity. 

 The total sensation is greater in amount when light falls on a large 

 area of the retina than when it falls on a small area. 



The effect of a stimulus also varies according as it falls upon an 

 eye accustomed to the daylight (the light-adapted eye) or an eye 

 which has been attuned to the dark (the dark-adapted eye). Upon 

 going from the light into the dark, it is impossible at first to see any- 

 thing; but after a time the eye becomes adapted to the darkness, 



