SENSE ORGANS AND SENSATIONS 247 



Now a lens forms an image of an object because all the 

 rays of light from each point of the object are focused in 

 corresponding points behind the lens. This is shown in 

 Fig. 96, where all the rays diverging from 1 are focused 

 at 1', all those from 2 at 2', and those from intermediate 

 points of the object at intermediate points of the image. 



If the rays from each point meet in front of the retina and 

 then diverge before reaching the retina, the retinal image is 

 blurred; and the image is also blurred if the retina is so 



ch 



ch 



FIG. 96. Diagram showing the formation of an image on the retina 



Jf, 2, the object ; 1', 2' , the image of the object ; c, cornea ; i, iris ; I, lens ; v, vitreous 

 humor; w, sclerotic; ch, choroid; o.n., optic nerve 



near the lens that the rays from each point have not yet 

 come to a focus. The more convex the lens, the more will 

 the rays of light be bent; consequently we use the muscle 

 of accommodation (which makes the lens more convex) to 

 get clear images of near objects (see Fig. 95). 



10. Myopia, hypermetropia, and presbyopia. In the em- 

 metropic eye (Fig. 97, E) the distance between the retina 

 and the lens is such that light from distant points comes 

 to a focus on the retina without any active muscular ac- 

 commodation; to see near objects the lens is made more 

 convex. 



