408 



RADIATION OF HEAT. 



But the radiation of heat, independently of any power of transmission which 

 may reside in the air, is put beyond dispute by the fact that a thermometer sus- 

 pended in the receiver of an air-pump, when it is exhausted, is affected by the 

 solar rays direited upon it. 



The effects of the radiation of hot bodies prove that rays of heat exist unac- 

 companied by light. On the other hand, the calorific property which con- 

 stantly accompanies the solar rays, as well as the rays proceeding from flame, 

 would indicate that heat is a necessary concomitant or property of light. It is 

 ascertained also that the calorific principle exists with different degrees of 

 energy in lights of different colors. Sir William Herschel, being engaged in 

 telescopic observations on the sun, found that the colored glasses which he 

 used to mitigate the brilliancy of that luminary, in order to enable the eye to 

 bear its splendor, were cracked and broken in pieces by the heat which they 

 absorbed from the light which acted on them. This led him to investigate the 

 calorific properties of the different component parts of solar light ; and the ex- 

 periments which he instituted led to an important extension of the analysis of 

 light originally discovered by Newton. 



Let A, B, C, fig. 1, be a section of a glass prism cut at right angles to its 

 length, and let S, S, be a ray of light admitted through a small aperture in a 

 window-shutter, and striking the surface of the glass at S. It is a property of 

 glass, which is explained in optics, that when light enters it in this manner, 

 the ray is bent from its course, and instead of proceeding in the direction S, S', 

 as it would do. if it did not encounter the glass, it is deflected upward in an- 



other direction, forming an angle with its original course. Now it is found 

 that the ray thus bent upward does not continue to form one line of white 

 light as before, but it spreads or diverges, and if received on the screen, instead 

 of illuminating a single spot, as it would do if it were not intercepted by the 

 prism, it covers an extended line on the screen from V to R, and the length 

 of this line increases if the screen be moved from the prism, and decreases if 

 the screen be moved toward the prism ; a necessary consequence of the di- 

 vergence of the rays issuing from the prism. It is also observed that this line 

 of light thus produced on the screen, is not a uniform white light like the spot 

 which would be jjroduced on a screen held between A, B, C, and the window- 

 shutter. On the other hand, an appearance is produced of a regular succession 

 of brilliant colors, the highest color, V, being violet, the next below this, indigo, 

 which is succeeded by blue, green, yellow, orange, and finally red, in regular 

 succession, each color occupying a certain space on the line of light. This 

 effect is commonly called the prismatic spectrum, and it depends upon two facts 

 which are ascertained in optics, namely : first, that the ray of light, S, S, is 

 compounded of several distinct rays, which differ from each other in color ; 

 secondly, that the glass of the prism A, B, C, is capable of refracting or bending 



