xx VIL] VISION. 281 



More exact results are however gained by 

 Schemer's method ( 4, 5). 



8. Irradiation. 



Cut out two patches of exactly the same size, of 



white and of black paper. 



Place the white on a sheet of black and the 



black on a sheet of white paper. 



Place them some distance off and adjust the eye 



so as to throw them a little out of the range of 



accommodation. 



The white patch will appear larger than the 



black one. 



9. Blind Spot. 



Make a bold mark on a sheet of white paper, 

 place the sheet on the table, and, closing the 

 left eye, fix the axis of vision of the other, by 

 steadfastly looking at the mark at a distance of 

 about 10 inches. Dip a new quill-pen in black 

 ink and place it close to the paper near the 

 mark. Keeping the axis of vision fixed, and the 

 head at the same distance from the table, move 

 the pen outwards (towards the right) from the 

 mark. At a certain distance it will become 

 invisible ; mark this spot on the paper. Carry 

 the pen still farther outwards. It will again 

 come visible ; mark this spot also. The two 

 spots will indicate the outer and inner limits of 

 the blind spot. Similarly the upper and lower 

 limits may be traced, and with a little practice 

 an outline of the blind spot, shewing even the 



