VISION. 



199 



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

 that the distance o p shall equal x y. 



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. 



FIG. 26. 



FIG. 26. 



A contrivance for use in determining the refractive indices of 

 water and of glass. 



(1) O) 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 ap- 



