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



Is the power of altering the focus of the eye exceptional or 

 general ? and what is the extent of the change ? 



Can eyes, having distinct vertical and horizontal foci, be made 

 to adjust these foci to any particular distance at the same time ? 



Have the two eyes generally different focal lengths ? 



Does a good resolving power always accompany a good adjust- 

 ing power ? 



Are markings on a flat surface resolved better by one eye or 

 both? 



What is the appearance of a vertical line, and what of a hori- 

 zontal line and a point, when within or beyond focus ? 



Do the powers of the eye vary much with time of day, bodily 

 or mental fatigue, &c. ? 



After a few preliminary experiments, the following apparatus 

 and observations were adopted as the most suitable means of 

 eliciting the information sought. 



Upon a pair of parallel wooden rods, A, PL VI., a stage is made 

 to slide; the stage consists of two flat pieces of wood, B, C, at right 

 angles to each other — the vertical one, C, carrying a test-object, 

 and the horizontal one a taper, D. The light being on the stage 

 w r ith the test object, gives it a constant illumination, as they both 

 move together. The eye being placed at one end of the rods, 

 the object is between it and the light. The object consists of a 

 slip of glass coated with a thin film of silver or other materia], 

 upon which fine lines may be scratched so as to transmit light ; 

 upon this surface is drawn a cross consisting of fine double lines 

 such as shown at E, the width between the double lines being 

 varied according to the observer and the nature of the experi- 

 ment in hand. The focus is considered to be accurate when the 

 fine double lines are perceived to be double ; and the adjustment 

 is considered imperfect when the cross appears to consist of misty 

 lines or a multiplicity of lines. 



The test object for ascertaining the resolving power of the 

 eye consists of two lines diverging at a very acute angle, such as 

 shown at F, and supplied with an index showing how many 

 thousandths of an inch space there is between the lines at any 

 part of their length. This object being made to slide behind the 

 perforation in the upright portion of the stage, commencing at 

 the end where the space is greatest, it is pushed towards the 

 narrow end till the lines just cease to be distinguishable as two; 

 the number of thousandths is then read off as an indication of 

 the resolving power of the eye. 



This test-object, and the same mode of using it, was adopted 

 in experimenting upon the power of various eyes to change 

 focus: the resolving power having been ascertained for the 

 shortest comfortable focus, the same was tried for double and 



