26 LIGHT. 



to and farthest from the earth. Seven minutes are now allowed by 

 calculation, for the passage of light from the sun to the earth, and 

 one twenty fourth of a second for its passage, from pole to pole, of our 

 earth. L. u. K. 



A body cannot be seen through a bent tube, except by reflection, and 

 the shadows of bodies are exact copies of the form of the original. 



4. It moves in right lines ; never in curves ; if turned, it is always 

 at an angle. 



5. Its rays are mutually repellent, as they always diverge,* if mov- 

 ing uncontrolled ; as observed when they are let into a darkened 

 room, through a hole in the shutter especially when the dust is 

 raised in the room, so as to render the progress of the rays visible. 



6. IT OBEYS THE LAWS OF ATTRACTION. 



It is refracted in passing from one transparent medium into an- 

 other ; going obliquely from a denser into a rarer medium the re- 

 fraction is always from the perpendicular, and vice versa ; there is a 

 constant ratio between the sine of the angle of incidence, and that of 

 refraction. 



A piece of money being placed in a bowl, and the eye so situated 

 as just to lose sight of it, is rendered visible by pouring in water. 



A stick, standing out of transparent water, appears bent at the 

 surface. 



A river, or other transparent water, is deeper than it appears to 

 be, because the image of the bottom appears too high. 



7. The amount of refraction is proportioned directly to the density 

 of the body. 



Inflammable bodies refract in a higher ratio, and of course, inflam- 

 mable gases refract more than those that are not. At 32 Fahr. and 

 pressure 30, the refractive power of the following gases is as follows ; 

 Atmospheric air, .00000 



Carbonic Acid, .00476 



Azotic Gas, .03408 



Muriatic Gas, - - .19625 



Oxygen Gas, .86161 



Sub-carburetted hydrogen gas, - 2.09270 



Ammonia, 2.16851 



Hydrogen Gas, - 6.61436f 



In general, the refractive power increases with the density of the 

 body ; but inflammable bodies, hydrogen, phosphorus, sulphur, dia- 

 mond, bees-wax, amber, spirit of turpentine, linseed oil, olive oil, 

 camphor, &c. have a refractive power, from two to seven times 

 greater, in respect to their density, than most other substances. 



* Rays from the sun and fixed stars, although divergent, are regarded as parallel., 

 because the immense distance renders the angle of divergence indefinitely smalk 

 t Henry, Biot, Arago. 



