THE OPTICAL DEFECTS OF THE EYE. 13 



Pig. 7. A C represents an object in front of a convex lens, and 

 a c the inverted image behind the lens. Eajs diverging from the 

 point A and falling upon the lens L are brought to a focus at a; 

 rays from B are similarly focussed at h, and so on. In a similar 

 manner, diverging rays from every point in the object A C that 

 enter the lens are brought to a focus in the image between a and c. 

 "We will then have in the position of « c a distinct inverted image 

 of the object A 0. If this image is received upon a sheet of white 

 paper we can see it only upon its front surface ; but if it is received 

 upon thin oiled paper, or upon ground glass, we can see it from 

 behind ; and if, while viewing the image from behind, the ground 

 glass be removed, we can still see the inverted image (or at least a 

 portion) occupying the same position as the ground glass just occu- 

 pied — being suspended, as it were, in the air, and forming what is 

 called an serial image. In order to see this serial image under favour- 

 able circumstances, one eye only should be used, and should be in a 

 line with the lens and the object, and should be at least ten inches 

 behind the position of the inverted lens. 



Chapter II. — Optics of Normal Eye. 



The human eye, from before backwards, is about one inch in 

 diameter. Its transparent media are the cornea, aqueous humour, 

 crystaline lens, and vitreous humour. This combination, with the 

 convexity of the cornea, is equal to a convex lens having a focus of 

 about one inch (more accurately ^ of an inch.) 



When a normal eye is directed to a distant object ( j. e. in a state 

 of rest), parallel rays of light are brought to a focus upon the retina, 

 and a very minute inverted picture of the/ object is sharply defined 

 upon that membrane. If the sclerotic eoat be removed from the 

 back of the eye of an ox, and the eye be placed in an aperture of a 

 darkened room, with the cornea looking, for instance, towards the 

 opposite side of the street, an inverted image of the buildins's, &c. 

 in front of the aperture will be seen at the back of the eye. 



The impression that objects make upon the reMna, is conveyed 

 through the optic nerve to the brain, but in what manner this com- 

 municates to the mind a knowledge of the appearance of objects, is 

 more than we can tell. We can simply say with Potterfield, that 

 " Grod has willed it so." 



We are aware, however, that although the eye may be free from 



