Mr. B. S. Proctor on the Focal Adjustment of the Eye. 301 



focal length of the eye only rendered it less clear > this we may 

 take to be the explanation of the discrepancies in the Table of 

 Adjusting Powers. Though the eye is really focused to the 

 various distances at which observations are made, the focus is 

 more free from aberrations when the resolving power is greater 

 in proportion to the distance than it should theoretically be. 



When an observation is made with both eyes, we generally 

 find the results coincide with those of the best eye, the clear 

 impression apparently eclipsing the misty one. 



Different observers describe the appearance of a line out of 

 focus in various manners as follows : a faint band but with clear 

 edges — a line with misty edges — a band consisting of two or 

 more misty lines, sometimes nearly clear and not constant in 

 number ; the motions of the eyelid will frequently alter the 

 number. 



M. J. P. says a line within focus looks like multiple lines, and 

 beyond focus, like a line with softened edges. 



F. R. believes he can focus to any distance, but at long dis- 

 tances a line looks rough at the edges. 



W. W. P. finds that when beyond focus, the horizontal line 

 splits, and the vertical becomes misty. 



I find a dark line at about a foot distance, while the eye is 

 adjusted for parallel rays, looks like two indistinct spectra, the 

 order of the colours being blue, yellow, and blue, with a dark 

 band between each of the blues and the yellow. When slightly 

 within focus (say 9 inches distant with a focus for 1 foot), nar- 

 rower spectra of brighter colours are seen. The same is observed 

 less distinctly of a horizontal line. With a great discrepancy of 

 focus the horizontal line splits into several lines, which change 

 their shade and position with the involuntary winking of the eye- 

 lid. A spot at the distance of two or three yards, seen by both 

 eyes while a distant object is looked at, appears as four spots 

 thus •• •• ; but if seen at the same distance while a near 

 object is looked at (say 12 or 18 inches), it appears as four spots 

 thus : : — the greater distance between the spots being the 

 result of the degree of convergence of the optic axes, the shorter 

 distance being the result of a focal aberration. Of course the 

 spots seen in this way were not clear ; and when luminous points 

 were used instead of dark spots, the forms they took, being more 

 easily examined, were perceived to be characteristic. A star 

 seen with a short focus appears like a luminous ring with radia- 

 tions, the principal part of which spread in the vertical direction. 



Looking at a light through a pin-hole in a card held at a foot 

 or two distance while the focus was for parallel rays, the appear- 

 ance was that of a group of misty luminous spots spreading 

 principally in the horizontal direction. 



