300 Mr. B. S. Proctor on the Focal Adjustment of the Eye, 



with the left, temporarily clear at 19 ; with both eyes at once, 

 15 inches is his best focus, and that is evidently imperfect. 



F. S. with either eye sees both lines clearly at distances between 

 4 inches and a foot; with both eyes, at distances from 8J inches 

 to a foot. The vertical line seen with both eyes appears split at 

 distances less than 8^ inches. 



M. J. P. with the right eye, by an effort sees both clearly at 

 8 inches; with the left at 10, and with both at 8 to 11 J inches. 



F. R. with either or both eyes sees both lines clearly at 6 inches 

 and greater distances. 



W. W. P. with either or both eyes sees both lines clearly at 5 

 inches and greater distances. 



M. S. with either or both eyes sees both lines clearly at 10 to 

 12 inches and greater distances. 



Of nine observers, four have the best resolving power for 

 horizontal lines, one for vertical^ and four equally good for 

 either. The better resolving power generally coincides with 

 the shorter focus; but in one instance (C. B.) the horizontal line 

 had the longer focus and the better resolving power. 



Theoretically, at double the distance from the eye the lines 

 should have double the space between them to be resolvable ; in 

 most cases less than double the space is required at double the 

 distance; in some cases the proportion is preserved; and in two 

 (T. P. B. and M. J. P.) I found it reversed for the horizontal 

 lines, though in all cases in which the measurements were 

 taken the observer considered the focus to be accurately ob- 

 tained at both distances. 



It is probable that there may frequently be a considerable 

 imperfection in the best focus which can be obtained by a given 

 eye at any distance, and that most observers will believe the 

 object to be in perfect focus when it is in the best focus which 

 they can obtain. It may thus happen that both vertical and 

 horizontal lines may be reported in focus if they are both at 

 once in as good focus as either can be placed in when taken 

 separately. It probably also will occur with some eyes that they 

 have a large range of focal adjustment, but that the degree of 

 aberrations will vary; thus there may be one adjustment more 

 free from aberrations than others. An eye capable of adjust- 

 ment with very little aberration for rays diverging at one foot 

 distance, may be capable of adjustment for parallel rays, but 

 then have a greater aberration ; thus when a star is looked at, 

 it may not be in perfect focus on account of the aberrations, 

 though any nearer object would, while that adjustment is re- 

 tained, be in worse focus. It is likewise probable that a line 

 would be reported in perfect focus if it were not very evidently 

 out of focus, and it were found that increasing or decreasing the 



