84 FRAGMENTS OF SCIENCE 



Kor does the optic nerve embrace tlie entire range even 

 of radiation. Some rays, when they reach it, are incom- 

 petent to evoke its power, while others never reach it at 

 all, being absorbed by the humors 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 

 afl'ect 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 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 impu- 

 nity; and at a certain definite temperature it emits a fee- 

 ble red light. As the current augments in power the 

 light augments in brilliancy, until finally the wire ap- 

 pears 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 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 



