1892.] 



connected ivith Cloudy Condensation, 



433 



is difficult now to say what the diffraction colours are. The convec- 

 tion currents in the flask now make themselves visible ; the air on 

 each side of the blue direct light is suffused with a variety of colours, 

 not now in regular vertical bands, but irregularly distributed and in 

 movement through the flask. 



Cause of the Colour. 



These experiments show that the colour produced by the small 

 drops of water depends on the size of the drops, and the depth of 

 colour on their number. But it is not so easy to follow the manner in 

 which the drops produce the colour. If we take the simplest case, we 

 can easily see how part at least of the colour is produced. In the 

 steam jet condensing dense, and colouring the transmitted light 

 yellow, part of the effect is no doubt due to some of the particles in 

 that form of condensation being so small that they reflect and scatter 

 the shorter waves of light, while they allow the longer ones to pass 

 through. The colour in this case is partly caused in the same way 

 as the yellow produced by small particles suspended in liquids, as in 

 Brucke's experiment with mastic, or as when silver chloride is 

 formed from a solution of the nitrate. The light reflected by the 

 liquids in these experiments is of a bluish tint, complementary to the 

 yellow light transmitted by them, and this blue light is polarised. It 

 has been found that when the steam jet is of a good yellow by trans- 

 mitted light, it reflects a good deal of a bluish light ; and further, 

 this blue light is polarised in the same way as the light from the 

 small particles in the experiments with liquids. 



While this explanation helps us so far to understand the manner in 

 which the yellow light is produced in steam jets, yet it fails to explain 

 the succession of colours seen in the expansion experiments, where 

 blue first appears, then green and yellow ; and when the expansion is 

 still further increased, the blue again returns to give place to a second 

 green and yellow. The most probable explanation of these colour 

 phenomena is that they are produced in the same way as the colours 

 in plates, somewhat after the manner Newton thought the colour of 

 the sky was produced . The order of succession of the colours in thin 

 plates is the same as in these condensation phenomena. As no white 

 follows the first blue, it seems probable that the first spectrum, or 

 order of colours, is not observed ; that the two generally seen are 

 the second and third. 



Some experiments were made with a glass tube receiver, in place of 

 the metal one, to see if there were any coloured light reflected in these 

 expansion experiments of the same kind as is seen under certain con- 

 ditions in the steam jet ; but no such colours have been observed. It 

 is possible they may be present; but, owing to the great amount of 



