THE SENSES 929 



or a foreign body in any of the refractive media and no eye is 

 entirely free from relatively opaque spots can be detected, and its 

 position determined by the shadow which it casts on the retina when 

 the eye is examined by a pencil of light proceeding from a very small 

 point. Let a diaphragm with a small hole in it be placed in 

 front of the eye at such a distance that a pencil diverging from the 

 hole will pass through the vitreous humour as a parallel beam, 

 equal in cross-section to the pupil (Fig. 407), and let the aperture be 

 illuminated by focussing on it the light of a lamp placed behind a 

 screen. The proper position of the hole will obviously be that of the 

 anterior principal focus of the eye i.e., the point at which parallel 



FIG. 407. In A the opaque 

 body o is in the plane of the 

 pupil. The position of the 

 shadow relatively to the bright 

 field is not altered when the 

 illuminating pencil is focussed 

 at P' instead of P. In B the 

 opaque body is in front of the 

 plane of the pupil. When 

 P is lowered to P', the shadow 

 moves towards the upper 

 edge of the bright field, and 

 appears to move downwards 

 in the visual field. When P 

 is raised, the shadow moves 

 towards the lower edge of the 

 bright field, and appears to 

 move upwards. In C the 

 opaque body is behind the 

 plane of the pupil. When P 

 is moved downwards to P', 

 the shadow moves towards 

 the lower edge of the bright 

 field, and appears to the 

 person under observation to 

 move upwards, and vice versa 

 when P is moved upwards. 

 The farther the opaque body 

 is from the pupil, the greater 

 is the apparent movement, 

 or parallax, of its shadow for 

 a given movement of the 

 source of light. 



rays passing from the vitreous into the lens, and then out of the eye 

 would be focussed. This method of examination of the eye is. 

 therefore, called focal illumination. Opaque bodies in the vitreous 

 humour will cast shadows on the retina equal in area to themselves, 

 The shadows of opacities in the lens and in front of it will be some- 

 what larger than the bodies themselves, since the latter intercept 

 rays which are still diverging ; but since the greater part of the 

 refraction of the eye occurs at the anterior surface of the cornea, it 

 is only the shadows of objects on the front of the cornea, such as 

 drops of mucus, which will be much magnified. Fig. 407 shows 

 diagrammatically how the shadows shift their position within the 



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