XLVI. Physiological optics, a. Determination of indices 



of refraction of water and of glass, b. Determin= 



at ion of focal distance of lenses, c. Verifi= 



cation of formula : -j- -f ,v = p. d. A 



simple dioptric system. 



a. Determination of the indices of refraction of water 

 and of glass. 



1. Appliances. Apparatus for determining the index of re- 

 fraction; a deep flat-bottomed water pan; a cube ot 

 glass 4-6 cm. in linear dimensions and polished on at 

 least two opposite sides. The two polished sides must 

 be absolutely parallel, whether the other sides are par- 

 allel makes no difference; centimeter rule and dividers. 



2. Preparation. A very convenient and sufficiently exact 

 apparatus for making the required determination may 

 be readily made as follows: 



(1) Take a carpenter's tri-square, constructed wholly of 

 iron; from the angle x (Fig. 26), where the gradu- 

 ated limb joins the body, measure off centimeters upon 

 the inner surface of the body and cut them in with a 

 file. 



(2) Locate on the inner edge of the graduated limb any 

 point, as y, 6 to 9 centimeters from the point x. 

 With files remove about ^ centimeter of the edge as 

 indicated in the figure, cutting deeply at z, so as to 

 leave a slender point at y as indicated. 



(3) Drill a hole in the inner surface of the body at o; 

 fit and drive a heavy brass or iron wire into this; 

 sharpen the upper end of the wire. The length of the 

 wire above the body must be two or three centimeters 



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