302 PHYSIOLOGY CHAP. 



with two small holes or slits very close together. The opposite end 

 of the second tube carries an illuminated slit in its centre. The 

 inner tube is provided with a scale the middle of wliich corre- 

 sponds to emmetropia. On looking through the instrument and 

 varying the distance between the illuminated slit and tlie two 

 apertures near the eye, it is easy to find the point at which a single 

 image appears. The point of origin of the single image is the far 

 point of clear vision, i.e. the static refraction of the dioptric system 

 of the eye examined. 



When the pupil is double, owing to congenital anomaly of the 

 iris, double vision occurs (monocular diplopia) when the eye is 

 not accommodated to the distance of the object ; single vision, 

 when it is accommodated. In these cases the double pupil acts as 

 an optometer. 



Volkmann first endeavoured to determine how frequently in a 

 given time he could accommodate the eye to the near point and 

 far point, and came to the general conclusion that accommodation 

 is rather a slow process ; this agrees with the fact that it 

 depends on the activity of smooth muscle fibres, which generally 

 react slowly. 



Hensen and Volckers found that accommodation for near 

 vision takes place more slowly than for distant vision, showing 

 that the contraction of the ciliary muscle is slower than its 

 relaxation. Vierordt stated that accommodation for near vision 

 required 1*18 r sec., for far vision 0'87 sec. Aeby and Eilhard 

 Schultze found that these figures varied considerably, but that the 

 contraction of the ciliary muscle was always slower than its 

 relaxation. Coccius obtained other results, and concluded that 

 accommodation was more rapid for near than for distant objects ; 

 Schmidt-Kimpler gave approximately the same figures for both : 

 2-72 sec. for near vision, 2'44 sec. for distant. Angelucci and 

 Auber, taking as their objective criterion the displacement of the 

 image reflected from the anterior surface of the lens, in passing 

 from near to distant vision, and vice versa, found no perceptible 

 time-difference. 



Bonders observed that contraction of the pupils does not occur 

 simultaneously with accommodation for near vision, but takes 

 place a little later. 



When the lens is absent (aphakia), either from a congenital 

 anomaly or after an operation for cataract, the refractive power of 

 the eye is deficient, i.e. there is a marked amount of hypermetropia, 

 which can be corrected by a convex lens of 10-11 D. We have 

 seen that this correction-value can be utilised to calculate the 

 total refraction of the lens, although it conies out lower than is 

 the case with other methods. 



It has recently been maintained by a number of ophthalmo- 

 logists that some degree of accommodation is possible, even when 



