204 



PHYSIOLOGY. 

 Fig. 8. 



situation b. In the same way, when we look into a river, 

 the pebbles appear to be raised in the water, and it looks 

 shallower than it really is. From this circumstance, persona 

 have often been drowned. 



23. Lenses. -A. lens is usually made of glass, its surface 

 being ground into some of the following shapes : 



The first of these on the left hand would be called a plain 

 lens ; the third, double convex ; the fourth, plano-convex ; the 

 fifth, double concave ; the sixth, plano-concave ; the seventh, 

 concavo-convex, fyc. Now, light in its passage through these 

 glasses, would be refracted in proportion to their greater or 

 less degrees of convexity or concavity ; but in passing 

 through a plain lens, like the first, the rays would not be 

 bent out of a straight course. For example, if you place 

 an object at the bottom of a tub of water, and place the eye 

 in a perpendicular direction over it, the object is seen pre- 

 cisely in its true situation ; but if you withdraw a short 

 distance, it will appear to be situated where it is not. 



