RETINAL STIMULATION. 257 



before you and look at it fixedly with one eye (the dis- 

 engaged eye being closed), then carry the point of a pencil 

 outwards from the marked spot until it passes out of view ; 

 mark where this occurs. 



Define the blind area in all directions ; the beginning of 

 the large vessels may be also noted. 



Measure the diameter of the outline thus obtained, and 

 the distance of the paper from the eye, and calculate the size 

 of the retinal image from the reduced eye. 



Discriminative power of the eye for detail. Place 

 a card on which parellel lines 1 mm thick are ruled at in- 

 tervals of 1 mm from each other upon a well illuminated wall, 

 and measure the greatest distance at which you are able to 

 recognise the lines distinctly from each other. By means of 

 the reduced eye, determine their distance from each other 

 in the retinal image. Compare the ascertained intervals 

 with the distanca which separates the outer segments of the 

 cones from each other in the fovea centralis. 



Periodic stimulation of the retina with white light. 

 The experiments are performed with discs of cardboard divided 

 into differently proportioned white and black sectors, &c. 

 The collaborator rotates the disc, whilst the observer stations 

 himself facing, and at a distance of about 2 '5 metres for a 

 disc of 20 cm diameter. Or the observer looks at the reflection 

 of the disc in a mirror at half the distance. He can then 

 operate the discs himself. 



1. White and black hemidiscs. Rotate at a gradually 

 increasing speed; just before the sensation becomes fused into 

 a silver grey there is a marked period of flicker. 



2. A sixth sector white. Rotate slowly at rather more 

 than one turn a second ; follow the retreating edge of the 



