XXVII.] VISION. 



passing through the chrome alum, viz. red anl 

 greenish blue, leave a rose colour. 



14. Region of normal colour-vision. 



Take small pieces of paper of various colours 



(about 10 mm. square). Fix the axis of vision, 



on a sheet of white paper, and place each 



of the coloured papers near the axis of vision, 



they will all be distinct. 



Place a red piece near the axis, and thc-n 



gradually move it towards the outside of the 



field of vision ; it will first become yellow, then 



a dull white, and will finally disappear. 



Kepeat this with a green piece, similar changes 



are seen, but the yellow is darker. 



Repeat this with violet, this becomes blue, then 



a dull white, and finally disappears. 



15. Positive After-Image. 



When waking in the morning, close and shade 

 the eyes for a minute or two, then suddenly 

 look at the bright window for a moment or two, 

 and then close and shade the eyes again. The 

 image of the window exactly corresponding to 

 the natural one, i.e. with the sashes dark and 

 the panes bright, &c., will last for some little 

 time. 



To succeed, the retina should be in rest before- 

 hand, and the exposure to the stimulus momen- 

 tary or nearly so. 



Or, in the evening, having closed and shad< d 

 the eyes for some time, suddenly look at a lamp 



