PR ACT 1C A L EXERCISES 



989 



this must be much weaker than the lens which has been removed, 

 for if the latter be placed in front of the eye, the image is formed a 

 little behind the cornea. 



(c) Replace the lens. Move the retina SD far back that the image 

 is focussed in front of it. This is the condition in the myopic eye. 

 Put a weak concave lens in front of the eye ; the image now falls 

 more nearly on the retina. Move the retina forward so that the 

 focus is behind it. This corresponds to the Jhypermetropic eye. 

 Put a weak convex lens in front ' 



of the eye to correct the defect. 



(d) Observe that a plate with a 

 hole in it, placed in front of 

 the eye, renders an indistinctly 

 focussed image somewhat sharper 

 by cutting off the more divergent 

 peripheral rays. 



(e) Fill with water the chamber 

 in front of the curved glass that 

 represents the cornea. The focus 

 is now behind the back of the eye 

 altogether. Refraction by the 

 cornea is here abolished, as is 

 the case in vision under water. 

 An additional lens inside the eye, 

 or a weaker one in front of it, 

 corrects the defect. Fishes have 

 a much more nearly spherical lens 

 than land animals, and a flat 

 cornea. 



(/) Fill the hollow cylindrical 

 lens with water, and place it in 

 front of the artificial eye. The eye 

 is now astigmatic. A point of light 

 is focussed on the retina, not as a 

 point, but as a line. The vertical 

 and horizontal limbs of a cross 

 cut out of a piece of cardboard 

 and placed in the path of the 

 beam of light cannot be both 

 focussed at the same time. 



6. Astigmatism (Regular). (i) 



FIG. 436. OPHTHALMOMETER, AS SEEN 

 FROM BEHIND THE PATIENT. 



B, blind for covering the eye not 

 being examined ; H, chin-rest ; A, A, 

 graduated discs on which radii of 

 curvature of the cornea in various 

 meridians are read off or their equiva- 

 lent in diopters ; E, eye-piece of 

 telescope ; C, milled head for raising 

 and lowering chin-rest ; F, milled head 

 for adjusting height of the ophthalmo- 



Look at a figure showing a number meter> and G for movi it horizon . 

 of lines radiating horizontally, tally back and forth 

 in intermediate 



n, graduated 



disc for giving the rotation of the outer 

 tube of the telescope and the black disc u. 

 In u are seen the two illuminated mires. 



vertically, and 



directions from a common centre. 

 First fix the figure at such a 

 distance that one can comfortably 



accommodate. If astigmatism is present, all the lines cannot be seen 

 w r ith equal distinctness at the same time, but they can all be succes- 

 sively accommodated for. Next, bring the figure to the near-point 

 of distinct vision for the horizontal and neighbouring lines. Probably 

 the vertical lines will be blurred and cannot be made as distinct 

 as the horizontal by any effort of accommodation. If the eye is 

 distinctly astigmatic, the difference will be marked. 



(2) Use the Ophthalmometer . A convenient form is shown in 

 Figs. 436 and 437. 



