568 MANUAL OF PHYSIOLOGY. 



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 

 luminous 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. 



It may be seen from the foregoing figure that if the retina, 

 which normally would lie at 2, were placed nearer the dioptric 

 apparatus, say at 1, or further from it, at 3, it would not meet the 

 exact point of the luminous cone, but would receive the rays either 

 before they came to a point or after they had diverged from it. 

 Thus, indistinct rings of light would be seen instead of one lumi- 

 nous point, and an image would be blurred and indefinite. 



From this it follows that the eye, when quite passive, can only 

 get an exact image of bodies which are placed at a certain dis- 

 tance from it, just as, for any given state of a camera, only those 

 bodies in one plane come into focus and give a clear picture on 



* 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 sub- 

 ject, and he must, therefore, refer those who are anxious to learn the 

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



