The Senses. 297 



opposite side of the lens at a point termed the focus; every 

 ray passing through the lens is refracted, excepting those 

 passing through the optical centre (see Fig. 28). If instead 

 of the rays being parallel, they diverge from a luminous 

 point, and that point be the focus, and pass through a 

 convex lens, then by passing through, they are rendered 

 parallel (Fig. 28). 



If rays of light be passed through a convex lens from a 

 point beyond its focus, after passing through the lens, 

 and so becoming refracted, they meet again instead of 

 becoming parallel, and the rays cross at some point, in such 

 a manner that the upper ray becomes the lower one, and 

 the lower ray the upper one ; in other words, the image 

 formed under these conditions is inverted (see Fig. 29). 



Fig. 29. 



Rays of light passing through a convex lens from /, at a point beyond 

 the focus/, cross at some point r, and invert the image (Landois 

 and Stirling). 



Applying these physical conditions to the eye, we find 

 that the rays of light passing into and through the cornea 

 are refracted ; they are further refracted passing through 

 the aqueous humour, considerably refracted passing through 

 the dense substance of the lens and vitreous humour, and 

 brought to a focus on the retina in such a way that the 

 retinal picture is upside down, and is only a miniature 

 though perfect representation of the object presented to 

 the eye (see Fig. -SO). 



The rays of light passing from X Y (Fig. 30) are re- 

 fracted and cross in the lens ; those rays, such as a and a', 

 pass through the lens without refraction, as they pass 

 through its optical centre ; the further behaviour of the 

 rays is shown in the diagram, and the method by which 

 the inversion of the picture is produced. The angle 



