RADIATION 



By John Tyxdall." 



I. — VISIBLE AXD INVISIBLE RADIATION. 



Between the mind of man and the outer world are interposed the nerves of 

 the human body, which transhite, or enable the mind to translate, the impres- 

 sions of that world into facts of conscit)usness and thought. 



Different nerves are suited to the perception of ditl'erent impressions. We do 

 not see with the ear, nor hear with the eye, nor are we rendered sensible of 

 sound by the nerves of the tongue. Out of the general assemblage of physi- 

 cal actions, each nerve, or group of nerves, selects and responds to those for the 

 perception of which it is specially organized. 



Tlie optic nerve passes from the brain to the back of the eye-ball and there 

 spreads out, to form the retina, a web of nerve filaments, on which the images 

 of external objects are projected by the optical portion of the eye. This nerve 

 is limited to the apprehension of the phenomena of radiation, and, notwithstand- 

 ing its marvellous sensibility to certain impressions of this class, it is singularly 

 obtuse to other impressions. 



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 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 ra3'S. 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 atiect the optic nerve a certain 

 temperatui'e is necessary. A cool poker thrust into a lire 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 ciu-rent of electricity of gnidually 

 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 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 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 infi- 

 nite in variety) are bent or refracted from their straight course ; and as difi'erent 

 ravs are difierently refracted 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 ra3's are added to the emission. Augmenting the temperature still fur- 

 ther, yellow rays a})pear beside the orange ; after the yellow, green rays are 

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



* The Rede Lecture, delivered in the senate house, befoie the university of Caiiibridgf, 

 Eiiglaud, May IG, IbUa. 



