INVERSION OF THE IMAGE. 567 



extremity of it rays pass through, and are more or less bent by 

 the lens. It will be sufficient to follow the course of three rays 

 from the head of the arrow. One of these passes through the 

 centre of the lens, and leaves it in the same direction which it 

 entered, because the two surfaces at the points where it entered 

 and left may be regarded as parallel, and so cause no refraction. 

 The rays which do not pass through the centre are bent on 

 entering and on leaving the lens, so that they all meet at the same 

 point and there produce an image of the head of the arrow at b' '. 

 In exactly the same way the feather end of the arrow is produced 

 at b ; the position of the image of the object is thus reversed by 

 the light rays passing through the lens. 



FIG. 223. 



Showing the course of the rays of light from two luminous points to the retina. The 

 rays from the point o, on passing through the cornea, lens, etc., are collected on the 

 retina at 6. Those from a' meet at b', and thus the lower point becomes the upper. 



In a biconvex lens, with the two surfaces of the same degree of 

 convexity, the central point through which the rays pass without 

 being refracted is easily made out, as it is the geometrical centre 

 of the lens. This central point is spoken of as the optical centre. 

 With systems of lenses of varying convexity, and more than one 

 in number, as we have in the eye, where the rays of light are 

 bent at different surfaces, it is much more difficult to determine 

 the optical centre. However, by means of the measurements 

 made by Listing, two points close together are known, which may 

 be said to correspond practically with the optical centres of the 

 eye ; they lie in the lens, between its centre and posterior sur- 



