On the Colours of a Jet of Steam. 265 



which appeared blue in nearly every position in which it could be 

 viewed. Looking end-on from below, the steam jet caused that part 

 of the heavens obscured by it to appear feebly orange coloured. The 

 day was bright, but the sky at this quarter was overcast. On look- 

 ing through the jet of steam from below upwards, but in a direction 

 inclined about 11° to the axis of the jet, in which position a portion 

 only of the steam-cloud could be viewed by the direct light of the 

 clouds, the remaining portion being sheltered by the side of the win- 

 dow, one part of the jet appeared orange red, namely, that part 

 which transmitted the direct light of the clouds, while the other por- 

 tion was blue. The blueness of the jet increased with the above- 

 mentioned angle until the angle was perhaps 30°, after which the 

 blueness somewhat diminished, but was far from being extinguished 

 at 90°. 



By partly closing the cock of the boiler, and so discharging steam 

 from the jet of, perhaps, not a higher pressure than 10 lb. on the 

 inch, I could obtain a jet of steam, which, looking end from below, 

 was blue. It was rather difficult to obtain this blue jet, and when ob- 

 tained, it kept alternating with violet. On now viewing this blue jet 

 under an angle as before (192) of about 20°, it appeared reddish- 

 orange in colour ; this colour was not visible at almost any angle, 

 like the reflected blue (192). 



Looking end-on, and adjusting the pressure, I have occasionally 

 seen for a moment at a time a bright green jet ; also, and commonly 

 a blue-purple. In the reflected tints I am not sure that I have seen 

 any thing more than orange-red, violet, and blue. The transmitted 

 colour appeared in my experiments more intense than the reflected 

 tints. This, perhaps, has its explanation in the fact, that when 

 looking end-on, the eye receives light which has shone through a 

 columnar arrangement, whose length is much greater than its diameter, 

 while the reflected lights would only be seen by looking on the con- 

 vex surface of the columnar stream of particles. 



Prof. Forbes, after discovering the red colour of a jet of steam by 

 transmitted light, connected the red colour of the clouds with this 

 fact ; and the truth of this connection is beyond dispute. So far, 

 however, as I have been able to go the colours of the steam jet are 

 manifestly only instances of ordinary interference, greatly resem- 

 bling that produced by thin transparent plates ; the transmitted ray 

 being always complementary to the reflected. Thus in 192 the trans- 

 mitted light is red, as in Prof. Forbes's experiments, but the reflected 

 light is blue. It is therefore to be inferred that all the colours of 

 the clouds originate in interference caused by minute drops of water, 

 the size of which determines their colour ; while the blue jet (192) 

 is, I think, strfetly analogous to the blue sky * 







* Phil. Mag., Aug. 1852. 



