382 



pable of; but the horizontal line I see indistinctly with much less 

 distinctness than that with which I see any part of the figure at the 

 distance of 10 inches. At the distance of 12 inches, I see the hori- 

 zontal line with the greatest distinctness and best definition my eyes 

 are capable of; but the vertical line I see indistinctly with much 

 less distinctness than that with which I see any part of the figure at 

 the distance of 10 inches. It thus appears that my eyes collect to a 

 focus on the retina the rays which diverge horizontally at the distance 

 of 8 j inches ; and the rays which diverge vertically at the distance of 

 12 inches. Whilst seeing the vertical line with perfect distinctness 

 and definition at the distance of 85 inches, I cannot alter the adjust- 

 ment of the eye so as to see the horizontal line more distinctly and 

 the vertical one less distinctly ; and vice versa, whilst seeing the 

 horizontal line perfectly defined at the distance of 12 inches, I can- 

 not alter the adjustment of the eye so as to see the vertical line 

 more distinctly and the horizontal one less. 



In short, I find that I have no power of altering the adjustment of 

 my eyes. I see vertical lines with perfect distinctness and definition 

 only at the distance of 8| inches, and horizontal lines with perfect 

 distinctness and definition only at the distance of 1 2 inches, and both 

 vertical and horizontal lines simultaneously with medium distinctness 

 only at the distance of 10 inches. 



At the distance of about 7 inches I see the vertical line with medium 

 distinctness, but the horizontal line very indistinctly. 



At the distance of about 14 inches I see the horizontal line with 

 medium distinctness, but the vertical line very indistinctly. 



At a nearer distance than 

 7 inches I see both lines indi- 

 stinctly, but the vertical less 

 so than the horizontal. At 

 a further distance than 14 

 inches, on the other hand, I 

 see both lines indistinctly, 

 but the horizontal less so 

 than the vertical. 



If now I view two oblique 

 lines, botk of which are 

 equally strong and black, 



