MOVEMENTS OF THE IRIS. 705 



condition and the degree to which it exists may easily be determined by 

 noting the differences in the foci for vertical and horizontal lines, and it may 

 be exactly corrected by the application of cylindrical glasses of proper curva- 

 ture. Indeed, the curvature of a cylindrical glass which will enable a person 

 to distinguish vertical and horizontal lines with perfect distinctness at the 

 same time, is an exact indication of the degree of aberration. Regular 

 astigmatism, such as just described, may be so exaggerated as to interfere 

 very seriously with vision, when it becomes abnormal. This kind of aberra- 

 tion, however, which is dependent upon an abnormal condition of the cornea, 

 is remediable by the use of properly adjusted, cylindrical glasses. 



Irregular astigmatism, excluding cases of pathological deformation, 

 opaque spots etc., in the cornea, depends upon irregularity in the different 

 sectors of the crystalline lens. Instead of a simple and regular aberration, 

 consisting in a difference between the depth of the vertical and the horizontal 

 curvatures of the cornea and lens, there are irregular variations in the curva- 

 tures of different sectors of the lens. As a consequence of this, when the 

 irregularities are very great, there is impairment of the sharpness of vision. 

 The circles of diffusion, which are regular in normal vision, become irregu- 

 larly radiated, and single points appear multiple, an irregularity described 

 under the name of polyopia monocularis. Accurate observations have shown 

 that this condition exists to a very moderate degree in normal eyes ; but it is 

 so slight as not to interfere with ordinary vision. In what is called normal, 

 irregular astigmatism, the irregularity depends entirely upon the crystalline 

 lens. If a card with a very small opening be placed before the eye and be 

 moved in front of the lens, so that the pencil of light falls successively upon 

 different sectors, it can be shown that the focal distance is different for dif- 

 ferent portions. The radiating lines of light observed in looking at remote, 

 luminous points, as the fixed stars, are produced by this irregularity in the 

 curvatures of the different sectors of the lens. 



While regular astigmatism, both normal and abnormal, may be perfectly 

 corrected by placing cylindrical glasses before the eyes, it is impossible, in the 

 great majority of cases, to construct glasses which will remedy what has been 

 called irregular astigmatism. 



MOVEMENTS OF THE IRIS. 



There are two physiological conditions under which the size of the pupil 

 is modified : The first of these depends upon the degree of illumination to 

 which the eye is exposed. When the illumination is dim, the pupil is widely 

 dilated. When the eye is exposed to a bright light, the retina is protected 

 by contraction of the iris. The muscular action by which the iris is con- 

 tracted is characteristic of the smooth muscular fibres, as can be readily seen 

 by exposing an eye, in which the pupil is dilated, to a bright light. Con- 

 traction does not take place instantly, but an appreciable interval elapses after 

 the exposure, and a more or less gradual diminution in the size of the pupil 

 is observed. This is seen both in solar and in artificial light. The second 

 of these conditions depends indirectly upon the voluntary action of muscles. 



