vi DIOPTRIC MEGHAN rSM OF THE EYE 311 



When the screen is at R, at the focal point of the lens, almost 

 all the rays fall on it close to the principal axis, at 0. But if the 

 screen is brought in front of the focus, at R', the refracted rays 

 form a diffusion -circle represented by AB. A glance at the 

 figure, however, shows that they cannot in this case be distributed 

 uniformly over the entire surface of the diffusion-circle ; many of 

 the rays fall on the periphery of the circle (at A and B), while 

 very few cut the centre of it. The diffusion-circle is therefore 

 seen as a ring that is brighter at the periphery than in the 

 centre. 



If the cone of light is now replaced by a line of light 

 (by means of a stenopaic slit), the image thrown on the screen 

 appears as a line 'with luminous points at the two extremities 

 (A and B] and is not doubled. 



M. Besso (1912), on throwing the image of a bright line on a 

 screen with a lens of 10-12 D., and then bringing the screen in 

 front of the focal point of the lens, saw the image of the line 

 become simple. The phenomenon is explained by the super- 

 position of the more luminous parts of the diffusion-circles that 

 form the image of the line. 



Spherical aberration in the dioptric system of the human eye 

 is always associated, as we have seen, with a certain degree of 

 astigmatism and imperfect centring of the dioptric media ; the 

 course of the refractive rays is consequently less simple, and the 

 diffusion-circles are more complicated, according to the position of 

 the rays. This explains why the phenomena of monocular poly- 

 opia present individual differences, and why some people see the 

 line double, others triple, etc., under the above conditions. But 

 the determining cause is stimulation of the retina by diffusion- 

 circles. 



(/) Owing to the fact that the refracting media of the eye 

 consist partially of tissues formed of cells, it follows necessarily 

 that they are not perfectly homogeneous and transparent. The 

 rays of light that pass through them must undergo a certain 

 amount of dispersion, but under physiological conditions this is 

 not sufficient to blur the outlines of the retinal images. 



There may also be more or less circumscribed and diffuse 

 opacities in the different media, but these do not interfere with 

 the function of the eye so long as it is accommodated to distant 

 vision. They throw no shadow on the retina, but make the image 

 less luminous. They can be entoptically perceived as dark spots 

 when a card with a small central aperture, through which the 

 light of a candle penetrates, is placed in front of the eye at the 

 anterior focal point (about 13 mm. from the cornea). The rays of 

 the pencil of light that reaches the eye from the illuminated 

 aperture arrive at the retina in a parallel direction after refraction. 

 Under these conditions the shadows of the opacities are projected 



