14 THE OPTICAL DEFECTS OF THE EYE. 



disease, and the connection between the retina and brain in every 

 way perfect, if the optical mechanism of the eye be in any way defec- 

 tive so as to produce ill defined images upon the retina, — vision will 

 be indistinct, and that the distinctness or indistinctness of vision 

 will be in exact proportion to the distinctness or indistinctness of the 

 inverted picture. Hence the necessity of understanding the optics 

 of the eye in order to comprehend the pathology and treatment of 

 the numerous optical defects to which it is liable. 



Case 1. Let me here take an example. A few weeks ago a phy- 

 sician of this city sent a patient for my advice, fearing that he was 

 losing the sight of his left eye. Upon examination, I found that he 

 bad what we call "paralysis of accommodation " of that eye. 



He could see distant objects with perfect distinctness, but near 

 objects he was unable to define ; he could not read large type unless 

 the letters were very large, and several feet from the eye. The eye 

 was, in fact, simply passive, like a convex lens, or a camera-obscura 

 with the screen to receive the image immovably fixed at the principal 

 focus of the lens, and could only bring parallel rays to a focus on, 

 the retina. 



I found that by rendering the diverging rays parallel, bv means of 

 a convex lens, he could see near objects distinctly ; by placing a six 

 inch convex lens before that eye, he could read fine type at six inches, 

 with a 10 inch lens at ten inches, with an 18 inch lens at eighteen 

 inches, &c. &c. The 6 inch lens rendered the rays parallel that 

 diverged from the letters six inches distant, and these parallel rays 

 falling upon the eye were brought to a focus upon the retina. [A 

 6 inch lens does not increase the apparent size of letters one-half, 

 whereas this patient could not see letters ten times the ordinary size 

 at six inches, or any distance less than about two feet from the eye. J 

 The 10 inch lens rendered the rays parallel from objects ten inches 

 distant, and the 18 inch lens from objects eighteen inches distant. 



The eye was unable to bring diverging rays to a focus upon the- 

 retina; in other words it had lost the power of "accommodation." 

 (We can temporarily paralyse the accommodation of the eye by apply- 

 ing a strong solution of Atropine.) 



A normal eye differs from the glass lenses we have been describing 

 in the fact that it can, not only focus parallel rays upon the retina, but 

 also rays that diverge from objects as near as from four to six or eight 

 inches from the eye. When parallel rays fall upon a 1 inch convex 



