922 



A MANUAL OF PHYSIOLOGY 



m the mirror or of the observer's pupil. When the mirror is rotated 

 an illuminated point of the observed retina will move in the opposite 

 direction over the retina.* The light proceeding from this point 

 when the observed eye is emmetropic is so refracted by the lens and 

 cornea that it leaves the eye as a bundle of parallel rays in the 

 direction of the image of the source of light (L') (Fig 401) If the 

 image of the flame reflected by the mirror is situated on the principal 

 axis of the observer's eye, and if the pupils of observed and observer 

 are of equal size, all the rays coining from the observed retina will 

 fall on the observer's retina, and therefore the whole pupil of the 

 observed eye will appear light. If the mirror is now rotated so that 

 the image of the source of light moves away from the principal axis 

 and the illuminating rays are no longer in that axis, the illuminated 

 point will move in the opposite direction from the principal axis 

 and the light returning from the pupil of the observed eye will ajrain 

 issue in the direction of the image of the source of light. It can 

 then happen that none of the rays hit the observer's pupil and the 

 observed pupil wilt appear entirely dark. Or the direction of the 



U 



FIG. 401. PATH OF RAYS IN SKIASCOPV (SNELLEN). 



U, observed eye ; Be, eye of observer ; Sp, mirror ; L, source of light ; L', image 

 of the source of light ; A, A', principal axis ; P, P', pupils. 



rays may be such that a portion of them enters the observer's pupil, 

 the rest being interrupted by its border. In this case the part of 

 the observed pupil from which rays enter the observer's pupil will 

 appear light, while the rest is dark. From Fig. 401 it can be seen 

 that the light part of the observed pupil is on the opposite side of 

 the principal axis from the image of the source of light. If, therefore, 

 the image of the source of light moves to the right (by rotation of 

 a concave mirror to the right, or rotation of a plane mirror to the 

 left) the skiascopic appearance in the observed pupil moves to the 

 left i.e., in the opposite direction to the image of the source of light. 

 If the observed pupil is myopic i.e., if its far-point is between 

 the observer and the observed eye, rotation of the mirror so far from 

 the principal axis that only a part of the rays issuing from the 

 observed pupil enter the observer's eye, will cause the pupil to appear 



* When a concave mirror is rotated to the right, the inverted real mirror 

 image also moves to the right, and the illuminated point to the left. 

 When a plane mirror is rotated to the right, the virtual mirror image 

 moves to the left, and the illuminated point on the retina therefore to the 

 right. 



