392 PHYSIOLOGICAL PHYSICS. [Chap. xxix. 



horizontal meridian FAE. These meridians are 

 represented by the straight lines CD and FE inter- 

 secting one another. Now the rays of light through 

 the vertical meridian will come to a focus at i, and 

 those through the horizontal meridian will come to a 

 focus at 5. I is called the anterior focal point, and 

 5 the posterior focal point, and the interval 

 between them is the focal interval of Sturm. The 

 result of the two foci is that between I and 5 a 

 series of circles of diffusion is formed, each circle 

 having a shape dependent upon its position. To 

 understand the formation of these images let con- 

 sideration be limited to one set of rays passing 

 horizontally, represented by the line FE, and another 

 set passing perpendicularly to them, represented by 

 the line CD, and that they thus intersect one another 

 as represented by the figure CFDE. Now at I 

 the vertical rays come to focus at a point while there 

 is still an interval F'E' between the horizontal rays. 

 To one looking from the front straight on this position 

 the intersection of the rays would produce a figure 

 represented in the diagram, to which the dotted line 

 from i points, where F and E show the interval between 

 the still converging rays, and c and D show the 

 vertical rays having reached their focal point. Ex- 

 amine a new position of the intersecting rays nearer 

 5. Here the vertical rays, having met in their focus, 

 now diverge, still in their vertical plane; but they have 

 diverged only a little as yet, while the horizontal rays 

 have approached nearer to one another as they move 

 to their focus. That is represented in 2, where c and 

 D are the now diverging vertical rays, and F and E the 

 still converging horizontal rays, and the diffusion 

 image is oval. At 3 the vertical rays have diverged 

 still more, the horizontal have converged, and a circle 

 is the diffusion image. At 4 is represented a point 

 still nearer to the focus of F and E, and where the 



