LIGHT. 119 



crystal of brown quartz or tourmaline, the heat is transmit- 

 ted in a smaller proportion than when the beam passes along 

 the direction of the axis of the crystal. 



It is generally as far as I am aware, universally true 

 that, while light continues as light, even though reflected or 

 transmitted by different media, little or no heat is developed : 

 and, as far as we can judge, it would appear that, if a me- 

 dium were perfectly transparent, or if a surface perfectly re- 

 flected light, not the slightest heating effect would take place ; 

 but, wherever light is absorbed, then heat takes its place, af- 

 fording us apparently an instance of the conversion of light 

 into heat, and of the fact that the force of light is not, in fact, 

 absorbed or annihilated, but merely changed in character, 

 becoming in this instance converted into heat by impinging 

 on solid matter, as in the instance mentioned in treating of 

 heat, this force was shown to be converted into light by im- 

 pinging on solid matter. As, however, I have before ob- 

 served, this correlation of light and heat is not so distinct, as 

 with the other affections of matter. One experiment, indeed, 

 of Melloni, already mentioned, would seem to show that 

 light may exist in a condition in which it does not produce 

 heat, which our instruments are able to detect ; but some 

 doubt has recently been thrown on the accuracy of this ex- 

 periment ; probably the substances themselves through which 

 the light is transmitted would be found to have been heated. 



The recipient body, or that upon which light fringes, 

 seems to exercise as important an influence on ovr percep- 

 tions of light as the emittent body, or that from which the 

 light first proceeds. The recent experiments o. Sir John 

 Herschel and Mr. Stokes show that radiant impulses, which, 

 falling on certain bodies, give no effect of light, become lu- 

 minous when falling on other bodies. 



Thus, let ordinary solar light be refracted by t prism (the 

 best material for which is quartz), and the spectrum received 

 on a sheet of paper, or of white porcelain ; looking on the 



