190 LECTURES TO SCIENCE TEACHERS. 



meaning of that is this that supposing a ray of light is 

 passing from me, the whole of the ether which is set in 

 vibration is not vibrating backwards and forwards in any 

 particular direction, as is usually the case, but it is set 

 in a circular mode of vibration. As I have already said, 

 Principal Forbes found that by employing a Fresnel's 

 rhomb, made of rock-salt, at the proper angles, he was 

 able circularly to polarise heat also. This and several 

 other experiments I should have liked to show you, but 

 unfortunately I have none of my apparatus here. For this 

 one I am indebted to the kindness of Professor Guthrie, 

 but the result remains the same, that the application of 

 the undulatory theory to heat has been absolutely proved. 



Perhaps the only connecting link that was wanted has 

 been supplied in late years to show that wherever there is 

 light there is also heat. The stars radiate to us a large 

 quantity of light, and if we could show that the stars 

 radiate heat to us, it was said by some that this would be 

 a final and conclusive argument of the identity of heat 

 and light. This was taken up by several astronomers, 

 Mr. Stone at the Royal Observatory at Greenwich, Mr. 

 Huggins, and also Lord Rosse. They all found effects of 

 this kind ; and Lord Rosse, by means of the apparatus 

 which you see before you, and by two thermopiles used in 

 a differential manner, was able to compare the heat 

 radiated by the moon and the space round the moon, and 

 at the different phases of the moon, by which he was led 

 to very remarkable results ; so that the fact has been 

 abundantly proved that both the stars and the moon 

 radiate a large amount of heat to us. 



I wish to point out to you with regard to this diagram 

 that the form of the curves is entirely hypothetical. You 

 must not think that they are at all likely to be exact, 

 because we really know very little about the amount of 

 radiation from different parts of the spectrum ; it depends 

 very much on the absorption of the substance we employ. 

 Very likely the maximum ought to be moved closer to the 

 red end, but that is a question which we cannot settle 

 definitely. 



I have attempted in a very feeble way to pass through 

 the various researches which led chiefly during the first 

 half of the present century to the final identity of light 



