36 THE POPULAR SCIENCE MONTHLY. 



Although a ray when passing from one medium to another is re- 

 fracted at different angles depending upon its obliquity, yet the phe- 

 nomenon is governed by one law and capable of being expressed in 

 one formula. This is called the index of refraction^ and was discov- 

 ered by Willebrod Snell, a Dutch philosopher, about the year 1621. 

 Fig. 5 will illustrate it. A circle is described around the point 7, at 

 which the ray R is incident upon the refracting surface. As the angle 

 of the incident ray i? varies with the normal, the angle of the refracted 

 ray S will vary also. The law of refraction is, that the signs of these 

 angles as B! P\ /S'Pwill have a constant ratio. Each transparent sub- 

 stance has its index of refraction, and tables are given of these indices 

 for different substances in the books upon physics. 



Fig. 6. 



"^*^*.- 



Theort of Mirage. 



In order that a ray may be refracted, it is by no means necessary 

 that it should pass through bodies of widely different qualities, as from 

 gases to liquids, or from liquids to solids ; the effect may be seen in 

 passing from one liquid to another of a different density, as w^here 

 liquid bisulphide of carbon is covered with a layer of w^ater floating 

 upon its surface. The ray will then be seen to be bent on entering 

 the water, and still more bent on passing from the water into the 

 layer of bisulphide of carbon. In the same way rays of light passing 

 through layers of the atmosphere of different density, undergo suc- 

 cessive refractions. As the atmosphere varies in its density as we 

 ascend from the earth, the rays of the sun and stars in passing through 

 them are bent in their course, so that in point of fact we see them all 

 through crooked and varying paths. 



An appearance, as of water, is often seen in sandy deserts, where 

 the soil is highly heated by the sun. The observer sees in the distance 

 the reflection of the sky and of terrestrial objects, as on the surface 

 of a lake. Fig. 6 illustrates how this effect may be produced. The 

 air near the ground becomes so highly heated and rarefied that its 



