764 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



have two linear foci, at right angles to the meridians of greatest and least 

 curvature separated by a space in which a section of the cone of rays will be 

 first elliptical, then circular, and then again elliptical. This defect exists to a 

 certain extent in nearly all eyes, and is, in some cases, a serious obstacle to dis- 

 tinct vision. The course of the rays when thus refracted is illustrated in Fig. 226, 

 which represents the interior of a box through which black threads are drawn 

 to indicate the course of the rays of light. The threads start at one end of the 

 box from a circle representing the cornea, and converge with diiferent degrees 

 of rapidity in different meridians, so that a section of the cone of rays will be 

 successively an ellipse, a straight line, an ellipse, a circle, etc., as shown by the 

 model represented in Fig. 227. It will be noticed that this and the preced- 



FIG. 227. Model to illustrate astigmatism. 



Ing figure are drawn in duplicate, but that the lines are not precisely alike on 

 the two sides. In fact, the lines on the left represent the model as it would 

 be seen with the right eye, and those on the right as it would appear to 

 the left eye, which is just the opposite from an ordinary stereoscopic slide. 

 The figures are drawn in this way because they are intended to produce a 

 " pseudoscopic " effect in a way which will be explained in connection with 

 the subject of binocular vision. For this purpose it is only necessary to cross 

 the axes of vision in front of the page, as in the experiment described on page 

 758, for studying the relation between the focal, axial, and pupillary adjust- 

 ments of the eye. As soon as the middle image becomes distinct it assumes a 

 stereoscopic appearance, and the correct relations between the different parts of 

 the model are at once obvious. 



This imperfection of the eye may be detected by looking at lines such as are 

 shown in Figure 228, and testing each eye separately. If the straight lines 



