298 THE SPECIAL SENSES. 



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 

 accommodation, on account of the greater curvature of the lens. 

 As stated in a preceding paragraph the extent of accommodation 

 varies with age.. At 10 years the range is from infinity, when the 

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

 used. In this case, therefore, the refractive power is increased 

 from 64.5 D. to 78.5 D., since a distance of 7 ctm., T ^- meter, is 

 equivalent to -^ or 14 + D. The decreasing range of accom- 

 modation as age increases is expressed conveniently in tha number 

 of diopters which may be added to the refractive power of the 

 eye by the action of the ciliary muscle. 



The following table illustrates the usual range of accom- 

 modation for different ages : 



Range of accommodation 

 Years. in diopters. 



10 14 



15 12 



20 10 



25 8.5 



30 7 



35 5.5 



40 4.5 



45 3.5 



50 2.5 



55 1.75 



60 1 



65 0.7-) 



70 , 0.25 



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- 

 ing to the character of the refracting medium. The short, rapid 

 waves at the violet end are refracted the most and are brought to 

 a focus before the longer, red waves, so that the image shows 

 fringes of color instead of being pure white. This phenomenon 

 is known as chromatic aberration. Lenses used for scientific 

 purposes are corrected for this defect or made achromatic by a 

 combination of lenses of crown and flint glass so placed that the 

 dispersive power of one neutralizes that of the other. The eye 

 exhibits this defect, but not to such an extent as to be noticeable 

 in ordinary vision. If, however, an object is in focus when viewed 



