73 6 THE SENSES 



justed as if for a nearer object, and therefore the red surface appears 

 nearer, Experiment 13. 



Astigmatism. The formation of perfect images becomes impossible 

 when either of the refractive surfaces of the eye has unequal curvatures in 

 the different meridians. This defect is called astigmatism. It was first 

 discovered by Airy. An astigmatic eye cannot form a perfect image of a 

 luminous point, and since images of objects are built up of the images of 

 the infinite number of points on their surfaces it follows that all images are 

 distorted. Luminous points appear with imperfect or star-shaped radii 

 or haloes. If these radii overlap in an image they tend to neutralize the 

 diffuse stimulating effects and such regions appear more distinct, while in 

 the converse condition the images are more blurred, figure 465. 



This defect, which is generally present in a slight degree in all eyes, is 

 usually seated in the cornea, but occasionally in the lens as well. 



The plane of greatest curvature in the cornea is usually in the vertical 

 meridian, a fact which doubtless comes from the pressure of the eyelids 

 during development. If one looks at figure 463, A or B, with one eye, the 

 three iines in the radii of the figure will be seen with unequal distinctness. 

 Certain sets will stand out sharp and black and others dim and with indistinct 

 outlines, and if the astigmatism is great enough the three lines may not be 

 distinguished. Figures C and D of this series enable one to detect minute 

 traces of astigmatism with great accuracy. 



ABC 



FIG. 463. Astigmatic Charts. 



It is somewhat difficult to picture the rays from a luminous point in their 

 course through eyes which have this defect, but an examination of figure 

 464 will show their refraction in astigmatism. In this figure four of the total 

 sphere of rays diverging from the point L in the arrows are represented as 

 striking on the refractive surface of the eye a,tA,B, C, D, and being converged 

 toward a focus. The rays A , C, separated by a vertical line on the refractive 

 surface, are focused at f lt while the rays A , B, separated by the horizontal 

 distance on the refractive surface, are brought to a focus at/ 2 . Rays from the 

 point L, therefore, have two apparent focal points, one point composed of the 

 rays that strike the refractive surfaces in a horizontal plane, / 2 , the other of 

 rays that strike in a vertical plane,/. If the retina of the eye be placed at/j, 

 it will receive an image of a luminous point with indistinct horizontal halos 



