108 ACCOMMODATION. 



this distance (the " near limit " of vision) at first slowly, then 

 more rapidly lengthens, until at 50, nothing nearer than 

 1 2 inches ; at 60, nothing nearer then 24 inches can be 

 defined. Whence it results that, inasmuch as the form of 

 the lens when unaccommodated remains nearly the same, 

 the range of adjustment in advanced life is exceedingly 

 small. The term myopia is applied to the condition in 

 which the lens is too convex for parallel rays, even when 

 the eye is atropinized ; hypermetropia, to that in which it 

 does not become convex enough for parallel rays, even 

 when the tensor is in full action. In the former case the 

 defect must be compensated by concave, in the latter by 

 convex lenses. 



The methods used for measuring the limit of near vision, 

 are founded on an experiment known as Scheiner's. A 

 diaphragm having two minute apertures at a distance 

 less than that of the width of the pupil, is placed immedi- 

 ately in front of the cornea, while the eye is kept fixed on 

 an object-point, at a sufficient distance to be distinctly 

 defined. If now the object is gradually brought nearer, it 

 is observed, that as soon as the " near limit " is passed, it is 

 seen double. The instrument used for making this experi- 

 ment with exactitude is called an optometer. 



For all investigations relating to accommodation, it is of 

 importance to be able to determine in the living eye the 

 convexity of the anterior surface of the lens, under different 

 conditions. This is done by measuring the apparent 

 diameter of an object seen reflected in it. The instrument 

 used is called an ophthalmometer. 



When the eye is contemplated by an observer so placed that the direction in 

 which he looks at it makes an angle of about 20 with the axis of the observed 

 eye, the image of any luminous object also placed in front of the eye, rays 

 from which form the same angle with the axis but on the opposite side, is seen 

 reflected in the middle of the cornea, and therefore, apparently, close to the 

 far border of the pupil. On the near side of this image a second appears 

 similar to it, but larger, feebler, and indistinct. When the observed eye is 

 accommodated, this image becomes smaller and a little more sharply defined. 



