10 THE OPTICAL DEFECTS OF THE EYE. 



Fig. 1, page 3, represents a 10 inch lens, at a distance of 20 inches 

 from an object, P. The lens is supposed to be divided into two 

 equal parts, of 20 inch focus each : the first half renders the diver- 

 ging pencil parallel, and the second half converges the parallel pen- 

 cil to a focus, at 20 inches from the lens ; t^— to = 2V* 



(Dr. Griraud-Teulon, of Paris, has ascribed the origination of the 

 above theory to Mr. J. Z. Laurence, of London, to w^hom we 

 are very much indebted, for his praiseworthy efi'orts to popularize this, 

 hitherto neglected, field of Physiological and Pathological Optics.) 



Let me next direct your attention to certain optical considerations, 

 which have a most important application, in the treatment of optical 

 defects of the eye. 



Tou may remember that in a former experiment, a 10 inch lena 

 was held ten inches from a white wall, so as to show the miniature 

 inverted picture of the window, &c., 20 ft. distant; and that when 

 the lens was brought to a distance of 60 inches from the window, it 

 was found that the image of the window was formed 12 inches be- 

 hind the lens, instead of 10 inches, and that at 10 inches, the image 

 was so indistinct as to be scarcely recognizable. 



Now suppose that a 12 inch lens be immovably fixed 12 inches 

 from the same wall, it will then be in a proper position to bring par- 

 allel rays to a focus on the wall, where it will form an inverted pic- 

 ture of the window, and objects at a distance beyond the window. 



If we now bring the flame of a lamp, for instance, to a distance 

 of 60 inches from the lens, no distinctly defined image of the flame 

 will appear upon the wall ; but if, by any means, we can render the 

 pencil parallel that diverges from the flame, the 12 inch lens will 

 then converge it accurately to a focus upon the wall, where we will 

 have an inverted image of the flame. 



Prom the knowledge that we have now obtained, we know that 

 a 60 inch lens placed in front of the 12 inch lens will render these 

 rays parallel. All that we have to do then is to combine a 60 inch 

 lens with the 12 inch lens : the 60 inch lens to render the rays par- 

 allel that diverge from the flame, 60 inches distant, and the 12 inch 

 lens to converge these rays to a focus, at the principal focal length 

 of the lens. This is exactly what we do in supplying old people 

 with convex spectacles. Their eyes are constructed to briug parallel 

 rays to a focus, on the retina ; but the rays from near objects are 

 too divergent to be focussed upon the retina without artificial aid ; 

 this deficiency is what we supply with suitable glasses. 



