VISION. 201 



tween the polished surfaces is not equal to x y, a point 

 y' may be located on the upper surface near the edge 

 of the glass block by making a dot with ink where the 

 line y x cuts the upper surface of the block. This line 

 is the normal. 



What is the index of refraction of the glass block 

 furnished by the demonstrator? 

 b. The determination of the focal distance of lenses. 



By means of a spherometer the radius of curvature (r) 

 of a lens may be determined.* 



If one knows the radius of curvature of a lens and the 

 index of refraction of the material of which the lens is 

 made he may compute the focal distance by using the 

 formula (1) F = for piano convex lenses, or (2) F=^7 TT 

 for bi-convex lenses. But there is an easier and more 

 direct method of determining the focal distance of a lens; 

 namely, by direct experiment. 



/. Appliances. An instrument such as is used in physical 

 laboratories for the same purpose or such a one as is 

 described under 2; several lenses ranging from 5 cm. to 

 50 cm. in focal distance. 



2. Preparation. A most satisfactory apparatus for this 

 purpose may be made by any student or demonstrator in 

 three or four hours. From thin pine boards construct a 

 simple box about 10 cm. square in cross section by 50 

 cm. in length. One end of the box should be closed 

 with a tightly stretched oiled paper for a screen, while 

 the other end may be closed with the same material of 

 which the rest of the box consists, the center of the end 

 having a circular aperture one or two centimeters in 



12 R 



*[ r=a! ir~r~a when a=spherometer reading, and l=the length of 

 one side of the equilateral triangle determined by<*ffe7 legs ,o| - | b f</'?^ 

 spherometer.] X / \^ \\\\\U// ' 



