SUNLIGHT AND ITS MEASUREMENT 1G7 



ular dispersions do, but reflect it by regular reflection. The 

 sky in consequence of the presence of dust assumes a whitish 

 tinge indicating that the light is regularly reflected from one 

 particle to another so that although it is reduced in quantity 

 by the time it reaches the observer the light is of much the 

 same quality as that originally incident upon the dust particles. 

 This, however, is not the whole story with regard to dust or 

 particles of water, such as may be contained in clouds. There 

 are a number of things to be taken into consideration in attempt- 

 ing to estimate these effects. In the first place many of the 

 dust particles may be electrical conductors and if this is the 

 case they would not behave with respect to light as do non- 

 conductors (as are the permanent gases of the air) since the 

 light is electro-magnetic in nature. Changes in surface, shape, 

 etc., of conducting dust particles will make large changes in 

 their optical properties. 29 



The whole question of the color of transmitted radiation 

 through suspended particles, large with respect to the wave 

 length of the radiation considered is complicated. If light is 

 incident on a body that is not absolutely opaque some light 

 is reflected at the surface, some is absorbed in the interior, 

 some is reflected from the interior back to the surface again 

 (again suffering some absorption) and the remainder is trans- 

 mitted. The light reflected from the first surface may be 

 of all incident wave lengths or the body may exhibit selective 

 reflection as is the case with copper, gold, some aniline dyes, 

 etc. 30 If the body has very little depth (as water droplets 

 in clouds, very fine dust particles, etc.) the internally re- 

 flected light is generally negligible. Reflected light is affected 

 not only by the internal reflections but also by the relative 

 index of refraction of the substance with respect to that of the 



29 See, for example, Garnett, J. C. M., Colours in metal glasses and in me- 

 tallic films. Phil. Trans. Roy. Soc. London 203A: 385-420. 1904. 



Idem. Colours in metal glasses, in metallic films, and in metallic solutions. 

 Phil. Trans. Roy. Soc. London 205A: 237-288. 1906. 



30 Wood, R. W., Physical Optics, Chap. 14; Preston, Theory of light; cited 

 in note 1. A brief exposition will be found in Burton, E. F., The physical 

 properties of colloidal solutions, Chap. 5. London, 1916. 



THE PLANT WORLD, VOL. 22, NO. 6, 1919 



