220 THE EYE 



Remembering that refraction takes place only at the plane 

 of junction of media of different densities, we see that it will occur 

 at (i) the anterior surface of the cornea, (ii) the anterior surface 

 of the lens, (iii) the posterior surface of the lens. At each of these 

 surfaces the incident ray will be bent towards the central axis, 

 and in the normal eye the result is to bring all incident rays 

 to a focus exactly on the retina. Thus a picture is produced. 



We have seen that refraction is always accompanied by a 

 certain amount of reflection, and this is the cause of the pheno- 

 menon known as Sanson's Images. If a candle is held at a short 

 distance from one side of the eye, the observer can distinguish 

 the images formed by each of the refracting surfaces. The image 

 produced by the cornea where the change in refractive index is 

 from 1 to 1-33 is much brighter than the images from the lens, 

 where the change is from 1 -33 to 1 -43 and from 1 -43 to 1 -33. 

 In the case of the two latter also there is a certain amount of 

 absorption of light by the media. The images from the cornea 

 and the anterior surface of lens are upright, that from the posterior 

 surface of lens is inverted. The radii of curvature of the refract- 

 ing surfaces are respectively, 8, 10 and 6 mm. Consequently the 

 images differ correspondingly in size. 



The combination of the three refracting surfaces constitutes 

 the physiological lens of the eye. 



Accommodation. 



Every one knows that in a photographic camera it is necessary 

 to adjust the distance between plate and lens in order to focus 

 sharply objects at varying distances. The eye, regarded as an 

 optical instrument, must suffer from this disadvantage, and it 

 is a matter of daily experience with us that near and far objects 

 cannot be seen clearly at the same time. How does the eye over- 

 come this difficulty ? The eyeball is rigid and the lens practically 

 fixed. No change in the relative positions of the latter and the 

 retina is possible. The adjustment, called accommodation, is 

 brought about by changes in the lens, so that the eyeball ha r 

 virtually a series of lenses of varying strength, from which it 

 selects the one most suited to the requirements of the moment. 



The lens suspended in the resting eye has not its natural 

 shape ; it is kept somewhat flattened by the tension of the 

 capsule. If this tension can be relaxed, the lens will become 

 more convex on account of its inherent elasticity. A mechanism 

 for bringing this about is present. The ciliary muscle is fixed 



