ELEMENTARY EXPERIMENTAL PHYSIOLOGY 113 



page 105), observe the near points of accommodation for a pin held 

 vertically and one held horizontally. Note if the distances are identical. 



EXPERIMENT II. Draw a rayed figure as follows : First, draw two 

 lines intersecting in the centre at right angles, and each about 5 cm. 

 long. Bisect each right angle by two other lines intersecting at 

 the same point, and each of these smaller angles bisect further by 

 four other lines. Bring this rayed figure to the near point of accom- 

 modation. Observe which of the lines can be distinctly seen, and 

 which are blurred. It will generally be found that the horizontal and 

 those adjacent will be clearly seen, when no effort will bring about 

 definition of the vertical. 



EXPERIMENT III. Using Kiihne's artificial eye, place in front of the 

 cornea the special glass trough (filled with water) designed for exhibit- 

 ing the nature of astigmatism. This has a plane surface posteriorly. 

 The anterior surface, however, is that of a cylinder, curved in horizontal 

 meridia but not in vertical meridia. Substitute for the arrow used in 

 earlier experiments as a source of light a stencilled cross, each bar being 

 about 5 cm. long and J cm. wide. Before introducing the astigmatic 

 lens, ascertain the position of the retinal screen necessary for definition 

 of the luminous object. Then place the lens in position. The image 

 will become changed. It will be found that the upper and lower edges 

 of the horizontal bar and the ends of the vertical bar are still distinct, 

 but otherwise definition of the vertical bar is absent. In order to 

 obtain definition of the vertical bar it will be necessary to move the 

 screen much closer, when a reversed effect will we seen viz., definition 

 of the vertical bar, its end, however, blurred ; the end of the horizontal 

 bar clear, but its edges altogether undefined. At no intermediate posi- 

 tion between the two foci can a clear image of the cross be obtained, 

 and it will be necessary in order to compensate for the presence of this 

 lens, convex in horizontal meridia, to introduce a second lens, plane on 

 one surface, and concave in horizontal meridia. This indicates the 

 method of correction of the defect in the human eye. 



4. Errors of Refraction. 1 In this division of the optical defects are 

 included the conditions of Myopia or short-sight, Hypermetropia or 

 long-sight, Presbyopia or the sight of old people. 



The normal eye in which the far point of accommodation is practically 

 infinity and the near point 20 cm. (8 ins.), is spoken of as emmetropic. 



Presbyopia. As a result of advancing age the power of accommoda- 

 tion for near objects may become diminished. Parallel rays are still 

 focussed on the retina, but the ciliary muscle is unable to bring about 



1 Properly speaking, astigmatism should be included in this section. We have 

 thought it best, however, to consider it in a separate section. 



H 



