KADIATION. 29 



Nor does the optic nerve embrace the entire range 

 even of radiation. Some rays, when they reach it, are 

 incompetent to evoke its power, while others never 

 reach it at all, being absorbed by the humours of the 

 eye. To all rays which, whether they reach the retina 

 or not, fail to excite vision, we give the name of invisible 

 or obscure rays. All non-luminous bodies emit such 

 rays. There is no body in nature absolutely cold, and 

 every body not absolutely cold emits rays of heat. But 

 to render radiant heat fit to affect the optic nerve a 

 certain temperature is necessary. A cool poker thrust 

 into a fire remains dark for a time, but when its tem- 

 perature has become equal to that of the surrounding 

 coals, it glows like them. In like manner, if a current 

 of electricity, of gradually increasing strength, be sent 

 through a wire of the refractory metal platinum, the 

 wire first becomes sensibly warm to the touch; for a 

 time its heat augments, still however remaining obscure ; 

 at length we can no longer touch the metal with im- 

 punity ; and at a certain definite temperature it emits 

 a feeble red light. As the current augments in power 

 the light augments in brilliancy, until finally the wire 

 appears of a dazzling white. The light which it now 

 emits is similar to that of the sun. 



By means of a prism Sir Isaac Newton unravelled 

 the texture of solar light, and by the same simple instru- 

 ment we can investigate the luminous changes of our 

 platinum wire. In passing through the prism all its 

 rays (and they are infinite in variety) are bent or re- 

 fracted from their straight course ; and, as different rays 

 are differently refracted by the prism, we are by it en- 

 abled to separate one class of rays from another. By 

 such prismatic analysis Dr. Draper has shown, that when 

 the platinum wire first begins to glow, the light emitted 

 is sensibly red. As the glow augments the red becomes 



