6 70 DEMONSTRATING REFRACTIVE EYE DEFECTS [Cn. XV 



Put the 3 or 4 diopter lens in place in the metal holder, and the 

 lantern slide of the radial lines (fig. 391) in the slide-carrier. 

 Light the lamp and focus the slide by moving it toward or from 

 the projection lens. Now introduce the 0.5 cylinder. Only the 

 horizontal lines will be sharp with the axis vertical or only the verti- 

 cal lines if the axis is horizontal. Put in front of the projection 

 lens the black disc with an aperture of 7.5 mm. The image will 

 be much improved. Remove this and put in place the disc with a 

 pupil of 2.5 mm. If now the light is well centered the entire circle 

 of the radial lines will be fairly good. The image will be rather 

 dim, however. 



Remove the small pupil and put in place the stenopaeic slit (fig. 

 39pB). Place the slit parallel with the axis of the cylinder and the 

 lines will all appear sharp. This is because the slit allows the 

 light to pass only along a line, thus eliminating most of the disturb- 

 ing rays from the unequal curvature. People with astigmatism 

 can partly overcome the trouble by narrowing the pupil and partly 

 closing the eye-lids so that objects are seen through a slit something 

 as in the experiment ( 93 la). 



932. Anisometropia or unlike refraction in the two eyes. 



This is not a rare defect. One eye may be normal and one astig- 

 matic, one with myopia and the other long sighted or normal, etc. 

 Where the two eyes are different, the efforts to get a correct image 

 are greatly hampered, for an accommodation which would give a 

 correct image in one eye will make the image of the other eye more 

 confused. 



When the differences in the two eyes are considerable, the image 

 of one eye is discarded, or the poor eye is turned aside (squinted) 

 to get it out of the way, and one gets along with monocular 

 vision. 



To make this demonstration in the most perfect manner there 

 should be two lanterns side by side, each projecting an image at the 



93 la. Preparation of the pupils and slit. These are easily made by 

 cutting out pieces of thin tin or other metal the size of the trial lenses and 

 boring the holes and cutting the slit. Metal is recommended because the im- 

 age of the crater must be focused on the pupil or slit, and paper or wood would 

 be burned by the absorbed energy ( 852.) 



