OPTICS 



521 



Expt. 93. Refraction. (Apparatus: Square bottle of eosin glycerin, 

 round diaphragm.) Pass a pencil of parallel rays into and through the 

 square bottle of liquid (a denser medium than air) lying on its side on a 

 block. Place the bottle so that its side shall be (A) at right angles to 

 the rays. Note that the rays pass into the denser medium and come out 

 again into the air unchanged in direction. Now (B) turn the bottle so 

 that the rays shall enter the bottle at the top of the side, the surface being 

 very oblique to the rays. The latter are now deflected, broken (refracted), 

 on entering the denser medium toward the perpendicular erected at the 

 point of incidence and on leaving the denser medium away from the 

 perpendicular. The two bounding surfaces of the denser medium being 

 parallel, the latter deflection is equal to the former and the rays leaving 

 the bottle are parallel to their course on entering it. Observe the re- 

 flection also, and the color of the efferent ray. 



Refraction by media with parallel sides. When a ray (e. g., A) passes into such a medium 

 at right angles to a side, it continues straight. When a ray (e. g., A') strikes at an angle, it is 

 bent (".broken") toward the perpendicular in the denser medium and away from it again on 

 emergence at E'. Thus, the emergent ray E' B' is parallel to A E C. 



Expt. 94. Prisms. Let parallel rays in a narrow pencil pass through 

 the prism and note their course. Work out the refraction-angles in 

 passing through the prism and in passing out of it. 



Place the prism in the direct sunlight and observe the solar spectrum 

 the rays having the highest vibration-number being refracted most. 



Demonstration of the Spectroscope. 



Expt. 95. Aberration and the Use of Diaphragms (Iris). (Appara- 

 tus: The three diaphragms, ground-glass, and a white card for a screen.) 

 Study the differences of the conditions with convex lenses with and with- 

 out a diaphragm to cut off the outermost rays. 



