460 



THE SPECIAL SENSES 



may be noted as follows : First, the retina a,nd lens of the 

 eye are fixed in position. The external aperture is pro- 

 tected by the transparent cornea and thus makes the 

 eye dust-proof. The air is replaced by the transparent 

 aqueous and vitreous humors, but this does not affect the 

 comparison, since these humors are transparent. They are 

 made necessary bj^ the fact that layers of the eye are not 

 stiff like the wood of the box and would collapse were it 

 not for the presence of these fluids. Finally, the lens 

 and retina are fixed in position, and the eye, therefore, must 

 resort to special means for producing pictures of near and 

 distant objects and bringing them all to focas upon the 

 retina. To understand this last process it is necessary that 

 we first understand how a lens produces images. 



How pictures are formed 

 by lenses. — Every poijit 

 of a luminous body trans- 

 mits light in the form of 

 rays. These rays pass 

 from the Imninous body 

 in straight lines and in 

 every direction. Wherever 

 a ray of light strikes it 

 produces an image of the 

 point from which it came. 

 In the ordinary course of 

 light rays, therefore, 

 images are formed all 

 over the receiving surface, 

 and the total effect is a 

 jumbling together of all these images so that it is im- 

 possible to detect anything but a mass of light. (See 



Fig. 217 — Scattered images produced by 

 rays distributed all over tbe receiving 

 Burface. 



