delivering the Keith Medal 409 



demonstrations. This he succeeded in doing in a way which 

 left upon his mind not the slightest doubt as to the truth of his 

 results ; the variations of temperature being so obviously dis- 

 played, as to prevent the slightest ambiguity as to the true source 

 from which they are derived. The instrument employed in the 

 research is the thermo-multiplier, of which the invention is due 

 to Nobili, though it has been greatly improved for experimen- 

 tal purposes by Melloni. Professor Forbes has likewise in- 

 creased greatly its power of indicating the more delicate effects 

 by employing a telescopic apparatus, which enables him to mea-. 

 sure a quantity of heat, perhaps not exceeding (rue-fifteen 

 hundredth part of a degree of Fahrenheit. 



That the Society may fully understand the nature of the 

 proofs afforded by Mr Forbes's experiments, reference must be 

 made to the correlative facts observed in the case of light. 



When light undergoes reflection from glass at an angle of 56%^ 

 its physical character is found to be thus far altered, that it re- 

 fuses to be a second time reflected by another plate of glass 

 placed to receive the ray at the same angle of BQ^^ if the plane 

 of incidence on the second glass be perpendicular to the plane of 

 incidence on the first. The light is then wholly transmitted by 

 the second plate. If the plane of incidence be the same for the 

 two plates, complete reflection takes place at the second plate. 

 This illustrates polarization by reflection. 



If a number of glass plates be used, and light traiismitted 

 obliquely through such a bundle of plates, it is in like manner 

 found, that the emergent light is wholly transmitted by a second 

 similar bundle placed parallel to the first, but is almost wholly 

 reflected, and therefore not transmitted, when the second bundle 

 is placed so that whilst the ray falls upon it at the same angle 

 as upon the first, the plane of incidence on the second bundle 

 being perpendicular to the plane of incidence upon the first 

 bundle. This is polarization by transmission or refraction. 



Lastly, It was observed before the close of the 17th century 

 by Huyghens, that certain bodies, as Iceland spar, endowed with 

 the property of double refraction, alter at the same time the 

 character of the light in the two refracted rays. So that, if two 

 sections similarly cut from a crystal of Iceland spar be placed 

 upon one another in conformable positions, or the respective po- 



