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LECTURES TO SCIENCE TEACHERS. 



arranged in a certain position no light can pass through 

 them, but if you turn one of them at a right angle then 

 the light can pass freely through both of them, and that is 

 the phenomena called polarisation. It was completely 

 explained by the undulatory theory of light and by that 

 theory alone, and it became desirable to see whether such 

 a thing could be done with heat from a low source of tem- 

 perature which was not luminous. Various experiments 

 had been made in the early part of the century to test this, 

 but with only negative results ; until hearing of the em- 

 ployment of this new form of instru- 

 ment for measuring temperature, viz. 

 the thermopile, the late Principal 

 Forbes employed the instrument in a 

 re examination of this question. He 

 had already tried the experiment 

 with other kinds of thermometers 

 with negative results, but on employ- 

 ing this thermopile with two pieces 

 of tourmaline he was enabled to show 

 the fact that there was polarisation, 

 and this having been once established 

 seemed to make it at once certain 

 that light and heat were absolutely 

 identical, because the source of heat 

 which he employed was a vessel of 

 boiling water which was non-lumin- 

 ous; but he continued to examine 

 the question further, and at last ob- 

 tained the means of measuring with 

 very great accuracy the amount of 

 polarisation. Here are two of the 

 instruments which were actually em- 

 ployed in these researches. They consist simply of a 

 cylinder and a piece of mica inclined at a proper angle 

 inside it. You know that if we take a large number of 

 plates of glass and lay them one over the other we can 

 reflect light at a cei*tain angle or transmit it at the same 

 angle, and the portion of light transmitted or reflected is 

 polarised ; and the larger the number of plates of glass, 

 the more complete is the polarisation. In order to get an 

 analogous phenomenon in the case of heat, he employed 



Flu. 4. 



