MEASURE OF THE POWER OF ACCOMMODATION. 837 



distance to diverge, so that they can be brought to a focus upon the retina. It 

 is remarkable that most infants are myopic at 'birth. This myopia, how- 

 ever, depends upon excessive curvature of the cornea and lens, and on ex- 

 cessive proximity of the lens to the cornea. As the eye grows this myopia dis- 

 appears. Either the too constant activity of the tensor of the choroid (in reading, 

 writing, etc.), or the continuous convergence of the eyeballs, whereby the external 

 pressure on the eyeballs is increased, is considered as the cause of the myopia 

 arising or increasing during school-life. 



3. The far-sighted (hyperopic, hypermetropic, presbyopic, over- 

 sighted, flat) eye (Fig. 288) is capable, when at rest, of focusing only 

 convergent rays on the retina (c). Distinct images can, therefore, be 

 formed only when the rays from objects are made convergent by a 

 convex lens, because parallel rays would come to a focus behind the 

 retina (at /). All rays coming from natural objects are either di- 

 vergent, or at most approximately parallel, never convergent. There- 

 fore, no hyperope can see distinctly when the eye is at rest, without a 

 convex glass. When the ciliary muscle contracts, slightly convergent, 

 parallel and finally even somewhat divergent rays may be brought to a 

 focus, by increasing efforts of accommodation. The far point is conse- 

 quently negative, the near point abnormally remote (more than 8 or 

 10 inches), while the range of accommodation is infinitely great. 



The cause of this defect is abnormal shortness of the eye, which is generally 

 the result of imperfect development in all directions. In addition the lens 

 becomes flattened in old age. The error is corrected by means of a convex lens. 



The far point of an eye is determined by bringing toward it objects that 

 subtend a visual angle of only 5 minutes (for example, Snellen's small letters, or 

 the medium type from 4 to 8 of Jaeger) and finding the point at which they 

 first become distinctly visible. The distance from the eye indicates the far point. 

 In obtaining the far point of a myope, the same objects (subtending a visual 

 angle of 5 minutes) are placed at a distance of 20 inches from the eye, and the 

 weakest concave glass is selected that will enable him to see the objects distinctly. 

 The near point is found by bringing minute objects (for example fine print) closer 

 and closer to the eye, until it becomes indistinct. The shortest distance at which 

 distinct vision is possible is designated the near point. 



The optometer may also be employed to determine the far and near points. 

 A small object, such as a pin, is moved to and fro over a scale, along which the 

 eye sights, as along a gun-barrel. The object is brought as close as possible and 

 is then removed as far as possible, so as to permit of distinct vision. The scale 

 shows directly the near and far points and also the range of accommodation. 



Other optometers are based on Schemer's experiment. By an arrangement 

 similar to that described the object is viewed through two small openings in a 

 card. When the object is within the near point, it appears double; and similarly 

 when it is beyond the far point. This may be readily understood from a con- 

 sideration of Schemer's experiments. The instruments of Porterfield and Stampfer 

 are constructed on this principle. In the latter a narrow, luminous slit, which 

 can be moved in a dark tube, is used as the fixing object. The optometer of 

 Th. Young and Lehot consists of a white thread stretched over a black scale. 

 The thread is observed through two small openings, and appears single and dis- 

 tinct when within the range of accommodation; within the near point, and beyond 

 the far point, however, it appears broken up into diverging lines. 



MEASURE OF THE POWER OF ACCOMMODATION. 



The range of accommodation, which is easily determined by in- 

 vestigation, does not of itself indicate the degree of force or the power 

 of accommodation. The measure of this is the mechanical work done 

 by the ciliary muscle. It cannot, of course, be determined directly in 

 the eve itself. It is, therefore, necessarv to take as its measure the 



