99 6 THE SENSES 



The phenomenon depends upon the interruption which the light pro- 

 ceeding from the observed retina experiences first at the margin of the 

 pupil of the observed eye, and then at the margin of the hole in the 

 mirror or of the observer's pupil. When the mirror is rotated, an illu- 

 minated 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 (I/) (Fig. 417). 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 coming 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 again issue in the direction of the image of the 



A' 



Fig. 417. Path of Rays in Skiascopy (Snellen). U, observed eye; Be, eye of ob- 

 server; Sf>, mirror; L, source of light; L', image of the source of light; A, A', 

 principal axis; P, P', pupils. 



source of light. It can then happen that none of the rays hit the 

 observer's pupil, and the observed pupil will appear entirely dark. Or 

 the direction of the 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. 417 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 eyef 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 light only 



* 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. 



