LESSON XL 



THE SENSE ORGANS (Continued) 

 VISION 



1. The Ametropic Eye. Astigmatism. Adjust the optical lantern 

 and box to form an emmetropic eye. Insert the diaphragm with the 

 2 mm. aperture in the opening of the optical lantern. Place a beaker 

 with water directly in front of the cornea. Note that the image now 

 simulates a vertical line. Close the top of the beaker by means of a 

 small piece of cardboard. Hold the beaker between your thumb and 

 index-finger and place it horizontally in front of the cornea. Note that 

 the image now simulates a horizontal line. Draw a diagram to show the 

 course of the refracted parallel rays. 



Repeat the preceding tests by holding a cylindric lens in front of 

 the cornea, first with its greatest curvature adjusted in the vertical 

 direction and then adjusted in the horizontal direction. By this means 

 may be imitated the conditions of "with-the-rule" and "against-the- 

 rule" regular astigmatism. It will be remembered that the astigmatism 

 which the physician is usually called upon to correct is due to a faulty 

 curvature of the cornea. In each case move the screen farther forward 

 and backward and note the changes in the shape of the image. Explain 

 these changes by means of a diagram. 



By means of a cylindric lens establish the condition of " with-the- 

 rule" astigmatism. Move the screen so as to obtain as distinct an 

 image as possible. Correct this condition by interposing in front of 

 this cylindric lens another one of the same refractive power, but ad- 

 justed in such a way that its greatest curvature comes to lie in the 

 horizontal plane of the cornea. 



Establish and correct the condition of "against-the-rule" astigmatism 

 in the same manner. 



Explain the fact that in the absence of a faulty curvature of the 

 cornea, all round luminous objects, such as lamps and stars, do not 

 appear round, but as radiate figures. 



2. Chromatic Aberration. Let the rays from the lantern emerge 

 parallel through the 2 mm. aperture. Hold a lens of 10 D. about 15 

 cm. in front of the. lantern, i. e., at a distance greater than the focal 

 distance of this lens. Place a white sheet of paper in the path of the 

 converging bundle of light at a distance of about 15 cm. from the lens. 

 Note the colors around the margin of the image. Correct this aberra- 

 tion by covering the edges of the lens with a circle cut out of cardboard. 

 How is chromatic aberration prevented in our eye? 



Make a pinhole in a card and place it upon cobalt glass. Close one 

 eye and with the other gaze at a gas flame through the pinhole. The 



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