750 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



view is derived from the circumstance that no fatigue is experienced from 

 prolonged direction of the Eye to distant objects, whilst the employment of 

 the visual power upon near objects for some time is accompanied with a sense 

 of effort, and is followed by fatigue. The movement which effects the change 

 of form of the crystalline lens, is performed in obedience to Volition, and is 

 guided by sensation ; yet we are not conscious of performing it, all that we 

 will being the result: and thus we have another apposite illustration of the 

 really automatic nature of what are termed " voluntary movements" gener- 

 ally ( 540). The time occupied in accommodating the Eye for near objects 

 is greater than that required for adapting it to view distant objects. Ac- 

 cording to Aby the period occupied in changing the accommodation from 17 

 to 4^ inches is about 2 seconds, whilst in changing it from 4? to 17 inches 

 only 1.2 seconds are required. Vierordt, however, 1 states that he was able 

 to effect the accommodation from 60 feet to 4i inches in about nine-tenths 

 of a second, and from 4i inches to 60 feet in about seven-tenths of a second. 

 Differences in age, and in amount of practice, are probably the chief causes 

 of such variations in the results of different observers ; accommodation for 

 near objects being always accomplished more slowly with the advance of 

 years. The adaptation of the Iris for variations in the intensity of light is 

 still slower. In healthy, or as Donders terms them, " emmetropic " eyes, 

 the limits of clear vision lie between two points, the "near-point" and the 

 "far-point." The former, very near the eye in infancy, gradually recedes 

 with advancing age ; and Fellenberg 2 has shown, that at 10 years of age it 

 is 2 inches distant from the front of the cornea ; at 20 years of age, of in. ; 

 at 30, 4| in. ; at 40, 6^ in. ; at 50, 12 in. ; at 60, 24 in. ; and at 70, 144 

 inches. The " far-point " for healthy eyes is infinite distance, or in other 

 words, the refractive media of healthy eyes in a condition of repose are 

 adapted to bring parallel rays to a focus on the retina. The extent of the 

 range of vision for each eye is very considerable, amounting, according to 

 Korn and Forster, 3 in the horizontal direction, to an arc of 130, and in the 

 vertical to an arc of 110 . 4 The actual rotation of the globe, or as Schuur- 



1 Henle and Meissner's Bericht, 1857, p. 547. 



2 Canstatt's Jahrosbericht for 1862, p. 157. 3 Ibid., p. 157. 



4 Landult (11 perimetro e la sua applicazione, Quaglino's Annal. d'Ottalmo- 

 logia, Gennajo, 1872) gives rather higher numbers, viz., 135 for the horizontal 

 and 120 for the vertical arc; and Uschakoft' (Reicheri and Du Bois-Reymond Ar- 

 chiv, 1870, p. 454) and Reich (Grafe and SLimisoh, Handb. der Augenheilkunde, 

 Bd. iii, 1874, p. 59) higher still, namely, in the horizontal meridian, max 142, min. 

 137; and in the vertical, max. 120, min. 114. The field of vision is smaller in 

 myopes a_nd rather larger in hypermetropes than in healthy or emmetropie eyes. 

 Helmholtz gives as average numbers derived from his observations the following 

 value to the varying optical constant and cardinal points. The numbers, where not 

 otherwise expressed, represent the distance from the anterior surface of the cornea in 

 millimetres : 



In accommodation 

 For distance. For iiear objects. 



Radius of curvature of cornea, .... 8 8 



Radius of curvature of anterior surface of lens, . 10 6 



Radius of curvature of posterior surface of lens, . 6 5.5 



Situation of anterior surface of lens, . . . 3.G 32 



" of posterior, 7.2 7.2 



Focal distance of lens 43.707 33 7S5 



Posterior focal distance of the eye, .... 19.875 17.7-VJ 



Anterior focal distance of the eye, .... 14.858 13274 



Position of anterior focal point^ .... 12.918 11.241 



Position of first chief point, 1.9403 2.0330 



Position of second chief point, 2.3563 2.4919 



Position of first nodal point, 6.957 6.515 



" second, 7.373 6.974 



" posterior focal point 22.231 20.248 



