On the Focal Power of Eyes for Horizontal and Vertical Rays. 481 



might, perhaps, be induced to make similar contributions. The 

 adjustment of the eyes for different distances being intimately con- 

 nected with the question of stigmatism or astigmatism, I have included 

 it in my analysis. 



If I view a vertical and 

 horizontal line, both equally 

 strong and black, I see them 

 with medium distinctness at 

 the distance of about 10 

 inches. 



At the distance of about 

 3|- inches, I see the vertical 

 line with greater distinctness 

 and better definition — the 

 greatest distinctness and best 

 definition my eyes are ca- 

 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 horizontal 

 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 afr the distance of 8J 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 1 2 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 12 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 indistinctly, 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 hori- 

 zontal less so than the vertical. 



If now I view two oblique lines, both of which are equally strong 

 and black, I see both legs with medium distinctness at the distance of 

 10 inches. 



