192 



RELATION OF HEAT AND LIGHT. 



luminous rays. However, in general, so far as the transmission of sunlight is 

 concerned, bodies which are absolutely transparent, or nearly so, are found to 

 arrest an extremely small portion of the calorific principle of the sun's light. 

 This effect, therefore, is generally consistent with the supposition that the cal- 

 orific principle is a quality of the solar rays. But numerous bodies are imper- 

 fectly transparent, or transparent only to lights of a particular color ; and in 

 this respect transparent objects bear an analogy to opaque ones. The color of 

 a transparent object when we look through it depends on the color of the light 

 which it transmits. Thus stained glass exhibits various colors according to its 

 quality when viewed from the interior of a window in which it is set. A piece 

 of blue glass admits a blue light to pass through it, but intercepts other colors. 

 Red glass, in like manner, allows a red light to penetrate it, but stops the pas- 

 sage of lights of other colors. The lights which are intercepted by partially 

 transparent objects, are partly absorbed by them and partly reflected. The por- 

 tion which is reflected is of that color which the object appears when viewed, 

 no source of light being behind it ; and the remainder is absorbed. Let us 

 suppose that the light which penetrates a piece of stained glass were mixed 

 with the light which is reflected, the mixture would not give the complete solar 

 light which strikes upon it ; the part which it absorbs would still be wanting ; 

 if that were added, the mixture .of the three would form white solar light. 

 Hence we see the reason-swhy a window of stained glass exhibits one set of 

 colors when viewed from the interior, and a different set of colors when viewed 

 from the exterior. When viewed from the interior the color which it transmits 

 is seen ; when viewed from the exterior, only the color which it reflects is ob- 

 served. 



To determine the effects of the sun's light in heating a transparent object, it 

 is necessary first to ascertain the color of the light transmitted through it, and 

 next the color of the light reflected by it. These two colors being subtracted 

 from the combination of color exhibited in the prismatic spectrum, the remain- 

 der will be the color of the light absorbed. 



A partially transparent object, therefore, will always absorb most heat when 

 the colors which its transmits and reflects are those which occupy the upper 

 portion of the prismatic spectrum ; for, in that case, the lights which it absorbs 

 are those which occupy the lower portion of the spectrum, and are the most 

 powerful in their calorific effects. 



Hence we see the reason why the colored glasses used by Sir William Her- 

 schel to mitigate the sun's light in his telescopes, were so frequently cracked by 

 the heat they absorbed. The splendor of the light in a large telescope, ren- 

 dered it necessary to use glasses of a very dark color, and consequently such 

 as absorbed the most calorific colors. 



The calorific power of the sun's rays may be exhibited in a very conspicu- 

 ous manner, by concentrating a large number of them into a small space, by 

 means of a burning-glass.^ Such an instrument is usually formed either of a 

 large concave reflector, by which the rays, falling on an extensive surface, 

 are reflected in lines which all tend toward one point, or by a large con- 

 vex lens of glass, which, when the rays pass through it, bend them, or refract 

 them, in directions converging all to the same point. In either case, the effect 

 of the rays is increased in the proportion which the magnitude of the point 

 into which they are collected bears to the magnitude of the reflector or the 

 lens. From experiments performed in this way by Count Rumford, it appears, 

 however, that no change in the heating power of individual rays is produced 

 by this means, and that the increased energy of their calorific action arises 

 altogether from a great number of them being concentrated in a small space. 



The heating power of the sun's rays, when collected by a burning-glass, far 



