182 SPECIAL PHYSIOLOGY 



(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 2 or 3 cm. 

 greater than the distance x y. Bend the point over so that distance 

 o p shall equal x y. 



3. Experiments and Observations. Place the instrument in the 

 water-pan; fill the pan, so adjusting it that both points p and y will 

 just touch the water, or rather almost touch the water, for the surface 

 of the water at y must be absolutely plane. If the point touch it 

 the surface will not be plane. 



(1) (a) Bring a small rule (R) into position and clamp it to the 

 limb of the instrument by means of heavy serre-fine forceps. So 

 adjust the rule that as one sights along its upper edge the points 

 a, y, and 3 seem to lie in one and the same straight line. Lift the 

 apparatus out of the water and lay it on the table, taking care not 

 to disturb the adjustment. 



(b) With dividers measure the distance from the point y to line 3. 

 This is the radius. Determine the point where the circumference 

 would cut the upper surface of the rule say, point b. 



(c) From this point determine the perpendicular distance to the 

 edge of the limb at c. 



(d) The line c y x is a normal to the surface of the water at the 

 point y. The angle i is the angle of incidence; the angle r is the 

 angle of refraction. Imagine a circle whose centre is at y and whose 

 circumference passes through b and 3. The line b c is the sine of 

 the angle of incidence. The line # 3 is the sine of the angle of 

 refraction. 



(e) What is the ratio of sine i to sine r, or 



rinei' 



(2) In the same manner determine the ratio of the sines of these 

 angles when the rule is so adjusted as to bring a' y 4 in apparently 

 one straight line. What is the ratio of sine i' to sine r', or 



sine i f 

 sine / 



(3) If the instrument has been carefully constructed, and if the 

 determination has been made with sufficient care, the ratios will be 

 found to be practically equal i.e., 



sine i sine i f 



sine r sine r' 



What is the constant ratio in the case of water? This constant 

 ratio is called the index of refraction and is conventionally repre- 

 sented by j". 



