844 WELLS's NATURAL PHILOSOrHT. 



The constitution of the solar ray may be compared to a bundle of three sticks, 

 one of which represents heat, another light, and a third the actinic principle. 

 "We know that these three principles exist in every ray of 

 j^""' ^^j. J^_ solar light, because we are able to separate them in a great 

 lar light con- degree from each other. Thus the luminous principle passes 

 prtaciples'f ^'^^ readdy through a transparent plate of alum, but nearly all the 

 heat is absorbed. Certain dark-colored bodies, on the con- 

 trary, allow nearly all the heat to pass, but obstruct the light. A blue glass 

 obstructs nearly all the light and heat of the solar ray, but allows the chem- 

 ital principle to pass freely ; while a yellow glass allows light and heat to 

 pass, but obstructs the passage of the chemical influence. 



When we decompose a ray of solar light by 



How are tho - . i ,i .1 . 



three princi- mcans 01 a prism, and throw the spectrum 

 fi^H^t "affected upon a scrcen, the luminous, the calorific, and 

 yapnsm ^^^ actlnic radiations will each assume a dif- 

 ferent position. All will be refracted by passing through 

 the prism, but in different degrees. 



The calorific, or heat radiations will be refracted least, and their maximum 

 point will be found but slightly thrown out of the right lino which the solar 

 ray would have traversed had it not been intercepted by the prism. The 

 heat diminishes with much regularity on each side of this line. 



The luminous radiations are subject to a greater degree of refraction ; their 

 point of maximum intensity being in the yellow ray, lying considerably abovo 

 the point of greatest heat. The light diminishes on each side of it, producing 

 orange, red, and crimson colors below the maximum point, and green, blue, 

 and violet above it. 



The radiations which produce chemical action are more refrangible than 

 either the calorific or luminous radiations, and the maximum of chemical 

 power is found at that point of the spectrum where light is feeble, and where 

 scarcely any heat can be detected. 



The positions in the spectrum of the heat and actinic radiations, which are 

 invisible to the eye, may be found by experiment. Thus, if we place a deli- 

 cate thermometer in the different rays of the spectrum (§ 686, Fig. 268), it 

 will be found that the indigo and violet rays scarcely affect it all, while tho 

 yellow ray, which is the most luminous, is inferior in heating action to tho 

 red ray, which, yielding but little light, possesses the greatest amount of heat. 

 If now, the thermometer bo carried a little below and just out of the red 

 ray, into the darkened space, it will exhibit the greatest increase in tempera- 

 ture, thus proving the presence of a heating ray in solar light, independent 

 of the luminous ray. In a like manner, by substituting a chemically prepared 

 surface, as a piece of photographic paper, for the thermometer, the presence 

 of a chemical ray can be proved in the darkened space at the other end of tho 

 Bpectrum, and near to the blue and violet rays. 



704. Those rays of solar light which are less refrangible than any of tha 



