312 AN AMERICAN TEXT-BOOK OE PHYSIOLOGY. 



spherical aberration of light has been explained. These three adjustments, 

 focal, axial, and pupillary, are so habitually associated in looking at near objects 

 that the axial can only by an effort be dissociated from the other two, while 

 these two are quite inseparable from one another. This may be illustrated 

 by a simple experiment. On a sheet of paper about 40 centimeters distant 

 from the eyes draw two letters or figures precisely alike and about 3 centimeters 

 apart. (Two letters cut from a newspaper and fastened to the sheet will answer 

 the same purpose.) Hold a small object like the head of a pin between the 

 eyes and the paper at the point of intersection of a line joining the right eye 

 and the left letter with a line joining the left eye and the right letter. If the 

 axes of vision are converged upon the pin-head, that object will be seen dis- 

 tinctly, and beyond it will be seen indistinctly three images of the letter, the 

 central one being formed by the blending of the inner one of each pair of 

 images formed on the two retinas. If now the attention be directed to the 

 middle image, it will gradually become perfectly distinct as the eye accommo- 

 dates itself for that distance. We have thus an axial adjustment for a very 

 near object and a focal adjustment for a more distant one. If the pupil of the 

 individual making this observation be watched by another person, it will be 

 found that at the moment when the middle image of the letter becomes distinct 

 the pupil, which had been contracted in viewing the pin-head, suddenly dilates. 

 It is thus seen that when the axial and focal adjustments are dissociated from 

 each other the pupillary adjustment allies itself with the latter. 



The opposite form of dissociation — viz. the axial adjustment for distance 

 and the focal adjustment for near vision — is less easy to bring about. It may 

 perhaps be best accomplished by holding a pair of stereoscopic pictures before 

 the eyes and endeavoring to direct the right eye to the right and the left eye to 

 the left picture — i. e. to keep the axes of vision parallel while the eyes are 

 accommodated for near objects. One who is successful in this species of ocular 

 gymnastics sees the two pictures blend into one having all the appearance of 

 a solid object. The power of thus studying stereoscopic pictures without a 

 stereoscope is often a great convenience to the possessor, but individuals differ 

 very much in their ability to acquire it. 



Range of Accommodation. — By means of the mechanism above described 

 it is possible for the eye to produce a distinct image upon the retina of objects 

 lying at various distances from the cornea. The point farthest from the eye 

 at which an object can be distinctly seen is called the far-point, and the nearest 

 point of distinct vision is called the near-point of the eye, and the distance 

 between the near-point and the far-point is called the range of distinct vision 

 or the range of accommodation. As the normal emmetropic eye is adapted, 

 when at rest, to bring parallel rays of light to a focus upon the retina, its far- 

 point may be regarded as at an infinite distance. Its near-point varies with age, 

 as will be described under Presbyopia. In early adult life it is from 10 to 

 13 centimeters from the eye. For every point within this range there will be 

 theoretically a corresponding condition of the lens adapted to bring rays pro- 

 ceeding from that point to a focus on the retina, but as rays reaching the eye 

 from a point 175 to 200 centimeters distant do not, owing to the small size of 



