RECTANGULAR PRISM. 



23. This remarkable property of transparent bodies may be illus- 

 trated experimentally by the apparatus represented in fig. 13 ; let 

 abed represent a glass vessel filled with 



water, or any other transparent liquid. 

 In the bottom is inserted a glass receiver, 

 open at the bottom, and having a tube 

 such as a lamp-chimney carried upwards 

 and continued above the surface of the 

 liquid. If the flame of a lamp or can- 

 dle be placed in this receiver, as repre- 

 sented in the figure, rays from it pene- 

 trating the liquid, and proceeding to- 

 wards the surface dc, will strike this surface with various obliquities. 

 Rays which strike it under angles of incidence within the limits 

 of transmission will issue into the air above the surface of the 

 liquid, while those which strike it at greater angles of inci- 

 dence will be reflected, and will penetrate the sides of the glass 

 vessel b c. 



An eye placed outside b c will see the candle reflected on that 

 part of the surface d c, upon which the rays fall at angles of inci- 

 dence exceeding the limit of transmission ; and an eye placed 

 above the surface will see the flame, in the direction of the re- 

 flected rays, striking the surface with obliquities within the limit 

 of transmission. 



24. A remarkable property of glass prisms, which proves of 

 great use in various optical instruments, depends on this property. 

 Let B, fig. 14, be a rectangular prism, the 



longest face of which is inclined at angles of 

 45 to the two rectangular faces. If a ray 

 of light, A B, enter one of the rectangular 

 faces perpendicularly, it will pass into the 

 glass without suffering any change of direc- 

 tion, and will encounter the surface B at an 

 angle of 45, which being less than 48 11', 

 the minor limit of possible transmission, it 

 will be reflected on issuing through the other rectangular sur- 

 face perpendicularly, will meet the eye as it would if B were the 

 only surface it had encountered, and the object from which the 

 ray has proceeded, and whose real direction is B A, will be seen 

 in the direction c B at right angles to B A. 



Fig. 14. 



25. IMAGES PEODTJCED BY LENSES. 



A lens is a circular plate of glass, the surface of which is 

 curved on one side or both. 



93 



