156 PHYSIOLOGY FOR NURSES 



verged to this principal axis forming the focus. Con- 

 cave lenses diverge the rays emitted and have no actual 

 focus. 



Prisms. "A prism is any transparent medium com- 

 prised between two plane faces inclined to each other." 

 A ray of light passing through a prism is not only 

 refracted, but is broken up into the different colors of 

 which light is composed. The undulatory theory teaches 

 that light is dependent upon vibrations of the ether. 

 The different colored rays vibrate at different rates 

 red, the slowest, at 458 millions of millions a second, 

 and violet, the swiftest, at a rate of 727 millions of 

 millions a second. Between these extremes the other 

 colors are interspersed, with varying rates for each, in 

 the order, violet, indigo, blue, green, yelloAv, orange 

 and red. 



ELECTRICITY AND MAGNETISM 



"Symmers' theory assumes that every body contains 

 an indefinite amount of subtile imponderable matter 

 which is called electrical fluid," formed by the union 

 of two fluids distinguished as positive and negative; 

 that in the natural state the two neutralize each other, 

 but that they can be separated by friction and other 

 means, one never appearing without the other, though 

 either may be present in excess of the other, so that a 

 body may be either positively or negatively electrified. 

 "Electricities of the same kind repel one another, and 

 electricities of opposite kinds attract each other." 

 Electricity circulates freely over some bodies and does 

 not circulate over others. The first are called conduc- 

 tors and the second nonconductors. 



Electricity may be frictional when produced by rub- 

 bing, but some bodies, like glass, give positive while 



