8io 



A MANUAL OF PHYSIOLOGY 



proper position of the hole will obviously be that of the anterior prin- 

 cipal focus of the eye, i.e., the point at which parallel 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 somewhat larger than the bodies 

 themselves, since the latter intercept rays which are still diverging ; 



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 down- 

 wards 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- 

 versd 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. 



FIG. 310. 



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. 310 shows diagrammatically how the shadows shift 

 their position within the bright field when the direction of the 

 illuminating beam is altered. Generally opacities in the vitreous 

 humour are movable, in the lens not. 



Purkinje's Figures. As was first pointed out by Purkinje, the 

 shadows of the bloodvessels in the retina itself, and even of the 

 corpuscles circulating in them, although neglected in ordinary vision, 

 may be recognised under suitable conditions, a conclusive proof that 

 the sensitive layer must lie behind the vessels (p. 812). 



If a beam of sunlight is concentrated on the sclerotic as far as 

 possible from the margin of the cornea, and the eye directed to a 



