DIOPTRICS OF THE EYE. 297 



come by using suitable convex glasses to aid the eye in focusing 

 the rays. It is obvious that in such cases the glasses need not be 

 used except for near work. 



Far Point of Distinct Vision. The normal eye is so adjusted 

 that parallel rays are brought to a focus on the retina. The far 

 point is therefore theoretically at infinity. Objects at a great 

 distance are seen distinctly, as far as their size permits, without 

 accommodation, that is, with the eye at rest. Practically it is 

 found that objects at a distance of 6 to 10 meters (20 to 30 feet) send 

 rays that are sufficiently parallel to focus on the retina without 

 muscular effort on the part of the eyes, and this distance, therefore, 

 measures the practical far point, punctum remotum, of the normal 

 eye. The rays at this distance are, in reality, somewhat divergent, 

 and that they produce a distinct image without an act of accom- 

 modation may be due to the fact that the rods and cones, the really 

 sensitive part of the retina, do not form a mathematical plane, but 

 have a certain thickness or depth. In the fovea centralis, for in- 

 stance, the cones have a length estimated (Greeff) at 85 // (0.085 

 mm.), and since the displacement of the focus of an object moved 

 from an infinite distance (parallel rays) to 6 or 10 meters from the 

 eye is less than this amount, the focused image would continue to 

 fall on some part of the cones without the aid of the mechanism of 

 accommodation. 



The Refractive Power of the Eye and the Range of Accom- 

 modation. The refractive power of lenses is expressed usually 

 in terms of their principal focal distance, a lens with a distance 

 of one meter being taken as the unit and designated as having a 

 refractive power of one diopter, 1 D. Compared with this unit, 

 the refractive power of lenses is expressed in terms of the recipro- 

 cal of their principal focal distance measured in meters; thus, 

 a lens with a principal focal distance of y 1 ^ meter is a lens of 10 

 diopters, 10 D., and one with a focal distance of 10 meters is 

 ^ diopter (0.1 D.). The anterior principal focal distance of 

 the combination of refractive surfaces in the eye is 15.5 mms. or 

 ^o meters. The reciprocal of this length of focus. -^ or 64.5 D., 

 expresses the refractive power of the eye under the normal con- 

 ditions in which the rays are refracted into the dense vitreous 

 humor. The anterior focal length of the cornea alone is given as 

 23.3 mm., which would correspond to a power of 42.9 D., while the 

 anterior focal length of the lens alone is equal to 50.6 mm. or about 

 20 D. In the combined system, therefore, the action of the cornea 

 is more important than that of the lens. Removal of the lens, as 

 in cataract operations, does not lessen the refractive power of the 



