DIOPTRICS OF THE EYE. 293 



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 ft (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 Surfaces in the Eye. 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 reciprocal of their principal focal dis- 

 tance measured in meters; thus; a lens with a principal focal dis- 

 tance of yV meter is a lens of 10 diopters, 10 D., and one with a focal 

 distance of 10 meters is y^ diopter (0.1 D.). The posterior prin- 

 cipal focal distance of the combination of refractive surfaces in the 

 eye is 22.7 mms. or f^ meters. The reciprocal of this length of focus, 

 ~~ or 44.05 D., expresses the refractive power of the eye under the 

 normal conditions in which the rays are refracted into the dense 

 vitreous humor. As compared with a lens in air, the refractive 

 power of the eye would be expressed by multiplying this figure by 

 the index of refraction of the vitreous humor, 44.05 X 1.3665 = 58.8 

 D., the figure usually given to express the total refractive power. 

 The refractive power of the crystalline lens alone is 16 D.; that of 

 the cornea, 43 D.; hence the latter surface has about two and one- 

 half times the refractive power of the lens. Removal of the lens, 

 therefore, as in cataract operations, does not lessen the refractive 

 power of the eye so much as when the action of the cornea is de- 

 stroyed, as happens in part when the head is immersed in water. 



Optical Defects of the Normal Eye. The refractive surfaces 

 of the eye exhibit some of the optical defects commonly noticed 

 in lenses, particularly those defects known as chromatic and spherical 

 aberration. White light is composed of ether waves of different 

 lengths and different rapidities of vibration, the shortest waves being 

 those at the violet end of the spectrum and the longest those at the 

 red end. In passing through a prism or lens these waves are re- 

 fracted unequally and are therefore more or less dispersed accord- 



