648 THE SENSES 



contraction and dilatation may be produced by a local action of certain drugs 

 which is independent of and probably often antagonistic to the action of the 

 central apparatus of the third and sympathetic nerves. 



The close coordination between the two eyes is nowhere better shown 

 than by the condition of the pupil. If one eye be shaded by the hand its 

 pupil will of course dilate; the pupil of the other eye will also dilate, though 

 unshaded, due to crossed reflex action. 



Defects in the Optical Apparatus. Under this head we may con- 

 sider the defects known as: i, Spherical Aberration; 2, Chromatic Aberra- 

 tion; 3, Astigmatism; 4, Myopia; 5, Hypermetropia. 



The normal or emmetropic eye is so perfect that parallel rays are brought 

 exactly to a focus on the retina without any effort of accommodation, figure 

 466. Hence all objects except near ones (in practice all objects at a distance 

 of twenty feet or more) are seen without any effort of accommodation; in 

 other words, the far-point of the normal eye at rest is at an infinite distance. 

 In viewing near objects we are conscious of the effort (the contraction of 

 the ciliary muscle) by which the anterior surface of the lens is rendered 

 more convex, and rays which would otherwise be focussed behind the retina 

 are converged upon the retina. 



Spherical Aberration. The rays of a cone of light from an object situated 

 in the field of vision do not all meet in the same point, owing to the greater 

 refraction of the rays which pass through the circumference of a lens than 

 that of those traversing its central portion. This defect is known as spherical 

 aberration. In the camera, telescope, microscope, and other optical instru- 

 ments it is remedied by the interposition of a screen with a circular aperture 

 in the path of the rays of light, cutting off all the marginal rays and allow- 

 ing the passage only of those near the center. Such correction is effected 

 in the eye by the iris, which forms a diaphragm to cover the circumference of 

 the lens, and prevents the rays from passing through any part of the lens 

 but its center, which corresponds to the pupil. The iris is pigmented to pre- 

 vent the passage of rays of light through its substance. The image of an 

 object will be most defined and distinct when the pupil is small, if the light 

 is abundant; so that, while a sufficient number of rays are admitted, the 

 narrowness of the pupil may prevent the production of indistinctness of the 

 image by spherical aberration. But even the image formed by the rays passing 

 through the circumference of the lens, when the pupil is much dilated, as in 

 the dark, or in a feeble light, may, under certain circumstances, be well defined. 



Distinctness of vision is further secured by the pigment of the outer sur- 

 face of the retina and of the posterior surface of the iris and the ciliary proc- 

 esses, which absorbs any rays of light that may be reflected within the eye, 

 and prevents their being thrown again upon the retina so as to interfere 

 with the images formed there. The pigment of the retina is especially im- 

 portant in this respect; for with the exception of its outer layer the retina is 



