802 PHYSIOLOGY. 



traction of which narrows the cone of luminous rays and conse- 

 quently the circles of diffusion. 



SCHEINEK'S EXPERIMENT. A card is taken in which two small 

 holes are placed close together. The card is held close to the eye, 

 and in front of it a needle is held. When you move the needle 

 nearer the card, and then farther from it, a position is found where 

 it is distinctly seen. If it be brought slightly nearer, the needle 

 appears double, and you obtain the double image. The explanation 

 is easily seen from the diagram, Fig. 350. 



e f represent the hole 3 in the card, a the point of the needle, & 

 a lens, and m n I a screen at varying distances from it. With the 



TS 



NA 



Fig. 352. Diagram Showing the Corneal Axis, U-E; the Optic 

 Axis, 0-A; the Visual Line, R-Y; the Line of Fixation, R-J; and 

 the Three Angles. (BALL.) 



The angle between U-E and the visual line R-Y is the angle Alpha, averaging 

 5 degrees. The angle between the optic axis (O-A) and the line of regard (R-J) 

 is the angle gamma. The angle between the optic axis (O-A) and the line of 

 vision (R-Y) is the angle beta. TS, Temporal side. NA, Nasal side. 



screen at n, a distinct single image of the needle is perceived, because 

 the rays e and f coincide and are focused at n n. At the position 

 m the image is blurred and double because the rays from e do not 

 coincide with those from /; while at I the image is also double 

 and blurred because the rays are intercepted after they have diverged 

 from their focus. Let & represent the refractive media of the eye, 

 and m n the retina. 



THE OPTIC Axis. This is a line which passes through 'the 

 nodal point and the center of the cornea. If prolonged backwards, 

 it falls upon the retina on the inner side of the yellow spot. 



THE VISUAL LINE. The visual line joins the macula lutea 

 with the point on which the eye is fixed. It passes through the cor- 



