THEORY OF ETHER 



167 



A table of refractive indices that is, of the ratio s | n ^ 



sin 0' 

 - for yellow sodium light from air is given below. 



The most convenient method of measuring the refractive 

 index of a body, if a solid, is to grind it to the form of a prism, 

 measure the angle of the prism by reflection of light with the 

 spectroscope, then measure the angle of minimum deviation 

 when light passes through the substance. If a liquid, it is 

 enclosed in a hollow prism, of which the sides are perfectly 

 plane parallel-sided pieces of glass, which will not themselves 

 cause any total deviation, and measure as with a solid. 



Table of Refractive Indices for the mean D line of Sodium. 



Diamond .... 2'42 

 Phosphorus .... 2-22 



Ruby 1-71 



Iceland spar (ord.) . . 1-658 

 (ext.) . . 1-486 



Topaz 1-61 



Flint glass . . . .1-6 

 Emerald .... 1'58 

 Quartz (ord.) . . . 1-544 

 (ext.) . . . 1-553 

 Rock salt .... 1-54 

 Citric acid . . . . . 1-53 

 Canada balsam . . .1-53 

 Felspar . . .1-52 



Potassium nitrate . .1-52 

 Potassium sulphate . . 1'51 

 Ferrous sulphate . . .1-5 



Crown glass . . . .1-5 

 Magnesium sulphate . .1-49 

 Fluor spar . . . .1-43 



Ice 1-31 



Carbon disulphide . .1-63 



Oil of bitter almonds . . 1-6 



Aniline 1-57 



Phenol 1-55 



Benzene . . . . T49 

 Glycerin . . . .1-47 

 Turpentine . . . .1-46 

 Sulphuric acid . . . 1-42 

 Alcohol (amyl) . . .1-4 



(ethyl) . . .1-36 

 Ether (ethyl) . . .135 

 Water . . . . . 1-33 

 Alcohol (methyl) . . . 1-33 



105. Explanation of Spectra, with Observations. In all 

 the previous observations we have paid more attention to the 

 luminous effects of radiation than to the thermal effects, but 

 generally simple experiments suffice to show that the observed 

 changes possess their thermal aspect. For example, the focus 

 of light is the focus of temperature ; the direction in which 

 light is reflected or refracted is the direction in which the 

 maximum heating effects are produced by the radiating centre 



