LESSONS IN PHYSICS. 71 



or the path of a single point in the light wave, is called a ray of 

 light. The smallest portion of light which can be separated for 

 experiment is called a beam of light; a collection of rays which 

 diverge from a point or converge to a point is called a pencil of 

 light. 



When light waves fall upon the surface of bodies some of them 

 are thrown back or reflected, and some of them are absorbed by 

 the bodies, so that they disappear as light waves, while in the 

 case of some bodies many of the light waves are allowed to pass 

 through them without much loss or change, the bodies being 

 called transparent, translucent, or opaque, as they allow differ- 

 ent quantities of light to pass. Transparent bodies allow so 

 much light to pass that objects can be seen through them dis-" 

 tinctly, as in the case of air, glass, water, etc. Translucent 

 bodies allow light to pass through them, but in such a scattered 

 condition that objects cannot be seen distinctly, as ground glass, 

 oiled paper, etc. Those objects are opaque which apparently ab- 

 sorb or cut off the .light waves so that nothing can be seen 

 through them. Transparent and opaque are relative terms, as 

 no body is wholly transparent or wholly opaque. 



The Velocity of Light. For all distances upon the surface of 

 the earth the passage of light seems to be instantaneous, but in 

 observing the eclipse of one of the moons of Jupiter it was found 

 to occur earlier than the time indicated by calculation. On 

 investigation it was found that when the earth is in that part of 

 its orbit nearest to Jupiter, the eclipse begins 16 minutes 36 sec- 

 onds sooner than it appears to begin when the earth is in the oppo- 

 site part of its orbit. From this it was concluded that light must 

 take 16 minutes 36 seconds to go across the earth's orbit. This 

 distance being known and divided by the number of seconds, 

 the velocity of light is found. The result is about 186,000 

 miles a second. Other investigations have led to the same con- 

 clusions. 



The intensity of light varies as the intensity of light in the 

 source. Again, the intensity of light varies inversely as the square 

 of the distance of the bodies emitting light, which is the same law 



