88 SENSE OF SIGHT. 



21. The optic ww, which comes from the brain, enters the 

 globe of the eye through the posterior part of the sclerotica, and 

 then expands itself out into a soft whitish membrane, called re- 

 tina, which envelops the hinder p irt of the vitreous humor. 

 Between the retina and the internal face of the sclerotica, we find 

 another membrane, generally colored black, called the c//oro///, 

 (tunica rliomidrx) It is this coat which is seen through the retina 

 and the humors of the eye when we look towards the bottom o 

 the organ, and which gives to the pupil the appearance of being 

 a black spot instead of a hole. 



Such ae the different parts which compose the globe of the 

 eye. Let us pass to the consideration of vision. 



22. The rays of light which leave an object at which we look, 

 penetrate to the retina and there form a small but very clear 

 image of that object. 



23. The manner in which the light acts in the interior of the 

 eye, is the same as in the optical instrument called a camera ob- 

 scura. The different transparent parts through which the lumi- 

 nous rays pass to get from the cornea to the retina, have the effect 

 of collecting the rays and concentrating them upon the retina. 

 It is the chrystalline lens especially that determines this concentra- 

 tion of light, and upon this phenomenon depends the formation of 

 images at the bottom of the eye. 



24. When the eye concentrates the light with too much force 

 we cannot see distinctly, except at a very short distance ; to this 

 infirmity is applied the term myopia, or short siylitcdness ; when, 

 on the contrary, the luminous rays are not sufficiently concentra- 

 ted in their passage through the eye, only distant objects are 

 distinctly seen, and this defect is called presbyopia or long-sight- 

 edness ; this feebleness in the refracting power of the eye, is a 

 consequence of old age, and is remedied by wearing convex 

 glasses before the eyes. To give short-sighted people a longer 

 vision, we must, on the contrary, employ spectacles with concave 

 glasses which scatter the luminous rays, and thus counterbalance 

 the too strong refracting f>rce of the eye. 



21. Where does the optic nerve enter the eye? What is the retina ? What 

 IK fnund between the retina and internal face of the sclerotica . Why does 

 the pupil se m to be a black spot instead of a h< le ? 



22. n what part of the eye are the imag s of objerts formed ? 



23. In what manner doe< light act in the interior of the eye? What effect 

 have the different p rts of the interior of the ey upon the light passing 

 from the cornea to the retina? What part especially determines the concen- 

 tration of light in the eye ? 



24. What is the consequence of a too great concentration of light by the 

 interior parts of the t ye ? -What is the cause of long sightedriess ? What 

 kind ol spectacles are required for short-sighted people? 



