566 MANUAL OF PHYSIOLOGY. 



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- 

 face. The path of the various rays may thus be exactly made 

 out* 



The rays which come from a distant luminous point and fall 

 upon the eye are refracted by the cornea and aqueous humor, so 

 as to be made convergent on their way to the lens ; they are then 

 further bent at the surfaces of the lens, so that they are brought 

 exactly to a point on the retina. That is to say, for distant lumi- 

 nous points, the retina lies exactly in the plane of focus of the 

 dioptric media of the normal eye. 



This convergence of the rays to a point on the retina is the 

 first essential in order to be able to see clear and distinct images ; 

 for if the rays from each point of a luminous body were not united 

 on the retina as points, the effects of the different rays from the 

 various points of a body would become mixed, and there would 

 be loss of definition of its image. 



The rays from any bright point which enter the eye through 

 the pupil may be imagined to form a luminous cone, the point of 

 which lies at the retina, and its base at the pupil. After their 

 union at the point of the cone the rays would diverge again if 

 the retina were not there to receive them. 



* The impossibility of making clear the important relationships, nodal 

 points, and other constants of the eye in a short text-book, and the deter- 

 rent effect exerted upon the mind of a junior student by brief incompre- 

 hensible statements, has induced the author to omit this part of the subject, 

 and lie must therefore refer those who are anxious to learn the cardinal 

 points of the eye to the more advanced text-books. 



