DIOPTRICS OF THE EYE. 317 



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 is taken as the unit and is 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 -^ meter is a lens of 10 

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

 ^ diopter (0.1 D.). In expressions of this kind it is assumed that 

 the lens is surrounded by air on both sides. In the case of the eye, 

 in which there is air on one side and liquid (humor) on the other, 

 it is necessary to distinguish between an anterior and a posterior 

 focal distance, according as the rays of light are conceived as 

 passing from the air into the eye and forming their focus in the 

 vitreous humor (posterior principal focus), or as passing in the 

 opposite direction and forming their focus in the air (anterior 

 principal focus). The posterior principal focal distance (reduced 

 eye) may be given as 20 + mm., while the anterior focal distance 

 is equal to 15 mm. (13 mm. in front of cornea). Compared with a 

 glass lens in air, the equivalent refractive power of the eye is equal 

 to 50 D (^r), reckoned for the posterior focal distance, or 66 D 

 (TJT), reckoned for the anterior focal distance. In the combined 

 system of cornea and lens the action of the cornea is more im- 

 portant than that of the lens. Removal of the lens, as in cataract 

 operations, does not lessen the refractive power of the eye so 

 much as when the action of the cornea is destroyed, as happens 

 for the most part when the head is immersed in water. The 

 total refractive power of the eye is increased by the act of accom- 

 modation, on account of the greater curvature of the lens. As 

 stated in a preceding paragraph, the extent of accommodation 

 varies with age. At ten years the range is from infinity, when the 

 eye is at rest, to 7ctm. when the maximum accommodation is used. 

 In other words, the increased curvature of the front of the crystal- 

 line lens in maximal accommodation adds to the refractive power 



