168 FRAGMENTS OF SCIENCE. 



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 temperature 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 be- 

 comes sensibly warm to the touch ; for a time its heat aug- 

 ments, still, however, remaining obscure ; at length we can 

 no longer touch the metal with impunity ; 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 

 lio-ht which it now emits is similar to that of the sun. 



O 



By means of a prism Sir Isaac Newton unravelled the 

 'texture of solar light, and by the same simple instrument 

 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 refracted from their 

 straight course ; and as different rays are differently re- 

 fracted by the prism, we are by it enabled 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 a pure red. As the glow 

 augments the red becomes more brilliant, but at the same 

 time orange rays are added to the emission. Augmenting 

 the temperature still further, yellow rays appear beside the 

 orange, after the yellow green rays are emitted, and after 

 the green come, in succession, blue, indigo and violet rays. 

 To display all these colors at the same time the platinum 

 wire must be white-hot : the impression of whiteness being 



