106 ELEMENTS OF SCIENCE 



dotted line CE be the direction of a ray passing straight 

 between E and 0. Then it is evident that if an opaque 

 vertical partition or septum, such as 0, be interposed 

 in the course of that straight line, the coin will be 

 invisible. If however the vessel be filled with water 

 a ray passing from C to S, just above the summit of 

 the septum, will be refracted to E for the following 

 reasons. A ray passing from air into water always 

 becomes more nearly perpendicular to i's surface and 

 one passing from water into air, always, as before said, 

 becomes more nearly horizontal to its surface, and so 

 the ray CS (from the coin to above the summit of the 

 septum) will be made more nearly horizontal and thus 

 pass to E and cause the coin to become visible. 



Bays which fall, or ascend, perpendicularly, are not 

 refracted at all, but the more oblique they are, the more 

 refracted they become, till they reach an extreme degree 

 of obliquity ; when they are no longer refracted, 

 but entirely reflected. This may be seen by looking 

 upwards or downwards through a glass vessel, very 

 obliquely, at the surface of water contained within it. 

 The water will then appear to have lost all its trans- 

 parency, and will reflect as an ideally perfect mirror 

 would do. For the laws of refraction the reader must have 

 recourse, as for all that is not quite elementary respecting 

 physical forces, to professed treatises on physics. 



Light passing through differently shaped transparent 

 media exemplifies supremely well the laws previously 

 stated with respect to the passage of heat through bodies 

 with differently shaped surfaces. When light passes 

 through glass which is flat (i.e., the opposite surfaces 

 of which are parallel) it is of course doubly refracted 

 the refraction undergone on entering the glass being 

 reversed when it emerges from it so that it regains its 



