VISION 



181 



III. PHYSIOLOGICAL OPTICS. 



Determination of the Indices of Refraction of Water and of 



Glass. 



1. Appliances. Apparatus for determining the index of refraction; 

 a deep, flat-bottomed water-pan; a cube of glass 4 to 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 parallel or polished makes no difference; centimetre rule and 

 dividers. 



2. Preparation. A very convenient and sufficiently exact appa- 

 ratus 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. 76), where the graduated limb joins the 

 body, measure off centimetres upon the inner surface of the body 

 and cut them in with a file. 



FIG. 76 



Apparatus for determining index of refraction. 



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

 as y, 6 to 9 cm. from the point x. With file remove about \ cm. 

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

 a slender point at y as indicated. 



