MIRAGE. 



301 



67 2 A ray of light proceeding from any given object along the 

 line EFG is totally reflected at F ; because the angle of incidence, 

 EFp ? is 67 J, and therefore greater than the critical angle from glass 

 to air (Expt. 337). For similar reasons the ray is again reflected 

 internally at G, so that it emerges and meets the eye at H, which 

 therefore sees the image formed at I on the paper where the draw- 

 ing is to be made. A lens is usually arranged at H so as to enable 

 the eye to distinguish clearly at once both the image and the pencil 

 point. Fig. 142 represents Am- 

 ici's camera lucida, consisting of 

 a right-angled glass prism, ABC, 

 and a plate of glass, DE, so 

 arranged that the line AB is per- 

 pendicular to DE. The incident 

 ray proceeding from an object at 

 F enters the prism, at G, is re- 

 fracted to H, where it is inter- 

 nally reflected, emerges by re- 

 fraction along IK, and at K is 

 reflected from the glass plate to 



Fig. 142. Amici's Camera Lucida. 



the eye at L; so that the image is seen at M, whilst the pencil is 

 also perceived through the glass plate DE. 



Expt. 339. The Mirage. The mirage is a peculiar natural 

 optical illusion noticed most frequently in hot sandy plains ; the 

 ground in the distance becomes invisible and in its place is seen, 

 apparently, a lake of water ; any objects standing up in the dis- 

 tance are usually seen inverted, as though reflected in the lake. 

 Fig. 143 represents the course pursued by a ray of light from such 



Fig. 143. Palm tree seen inverted by Mirage. 



an object (a palm tree); the air in contact with the heated ground 

 and the successive layers above it form a succession of media of 



