384 THE BODY AT WORK 



mind upon the sensations which it receives ; and not to physical 

 changes in sense-organs. Positive after-images are well-marked 

 appearances, although less common, perhaps, than the phe- 

 nomena of reversal of sensation of which we have just written. 

 On waking in the morning, one looks at the window ; shifting 

 the gaze to the ceiling, an after-image of the window appears, 

 just as one saw it, with bright panes and dark frame. The 

 " dark adapted eye," being exceptionally sensitive, yields the 

 same persistent positive after-image as the eye in its usual con- 

 dition yields, after being directed towards the sun at mid-day. 

 Movement-after-images can be explained only by referring 

 them to misdirection of judgment. If the gaze is fixed on a 

 rock close beside a waterfall, then shifted to a bank covered 

 with grass or bushes, the part of the bank which occupies the 

 lateral part of the field of vision appears to rush upwards, 

 reversing the movement of the water. When the gaze has 

 been fixed upon falling water a narrow stream sparkling in 

 sunlight a central strip of the field moves upwards, the 

 margins remaining stationary. If one stares at the spot on the 

 surface of a basin of water on which drops are falling from a 

 tap, and then looks at the floor, it is seen to contract towards 

 the spot looked at, reversing the movement of the ripples in 

 the basin. These observations reveal a fact of great import- 

 ance in the physiology of vision. It is, probably, impossible 

 truly to fix the gaze. The muscles of the eyeball keep the 

 retinal field in constant movement larger movements with 

 minute oscillations superposed. When, as in watching a 

 waterfall, movement has for a time taken a definite direction, 

 its cessation is judged to mean reversal. 



The anatomical unit of sensation is a cone. The fovea 

 centralis, the only part of the retina capable of receiving sensa- 

 tions sufficiently discrete for reading, contains cones alone. 

 If the gaze be directed but a very few millimetres on to the 

 white margin of the page, letters lose their form. In the fovea 

 the centre of one cone is 3-6 p distant from the centre of the 

 next. Two stars are visible as separate stars if they subtend 

 an angle of at least 60 seconds with the eye. Their images on 

 the retina are then 4 p apart. Parallel white lines ruled on 

 black paper, held at such a distance as causes them to subtend 

 angles of 60 seconds with the eye, appear not straight but 



