432 



Mr. J. Aitken. On some Phenomena 



[Apr. 28, 



gave the spectrum of the light, and the other half the light after 

 passing through the cloudy condensation. The conditions in the ex- 

 periment are too fleeting for satisfactory observation. The only thing 

 noticed was a darkening of the whole spectrnm, with a greater ab- 

 sorption at certain points than at others. When the light was bine, 

 in addition to the general r eduction in brightness, the red end was 

 more reduced than any other part, and there was also a very marked 

 shortening of the spectrum at this end. "When the colour was yellow, 

 the reverse was the case. The blue was almost entirely cut out, 

 while the yellow was far the brightest part of the spectrum. 



An examination has also been made of the diffraction colours as 

 seen in the halos surrounding bright lights. The most convenient 

 way tried of observing these colours was to use an ordinary glass 

 flask of IS cm. diameter, connected with the metal vacuum receiver, 

 as already described. For the source of light gas may be used, but a 

 better result is obtained with the light of the sky. In order to 

 observe these colours easily, the window should be closed, all but a 

 narrow vertical strip ; and it improves matters to have all surfaces 

 on each side of the opening painted black. When the air in the flask 

 is expanded, the vertical bands of diffraction colours are distinctly 

 seen on each side of the bright light. If now we keep the amount of 

 dust in the air constant during the experiments, we shall find, that on 

 opening the stopcock to the vacuum, receiver very slowly, we will get 

 the usual cloudy condensation, and that the diffraction colours will 

 be quite distinct. But if we repeat the experiment, and this time 

 open the stopcock very suddenly, so as to cause a rapid expansion, the 

 colours will be found to be very much improved, being far more 

 brilliant. This is due partly to the greater number of particles en- 

 gaged in producing the effect, but chiefly to the much more equal 

 size of the particles when they are suddenly developed than when 

 slowly grown. 



It is found that we must not have too many particles present, or 

 the diffraction colours will not be good ; their size does not seem to be 

 great enough to produce the phenomena. If, for instance, in place 

 of using the air of the room, we take into the flask air coming from a 

 small flame, the colour phenomena in the flask all change : when there 

 were few particles the light transmitted directly through them has 

 so little colour it is not noticed, while the diffraction colours are 

 fine; but with many particles the direct light becomes coloured, while 

 the diffraction colours are softened and have lost much of their bril- 

 liancy. When the particles are sufficiently numerous to cause the 

 directly transmitted light to be of a thin blue, the diffraction colour 

 next the blue light is nearly the complementary yellow, and this yellow 

 light extends to near the limits of the flask. If more particles be 

 added, the colour of the transmitted light becomes deeper blue, but it 



