526 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
its sensibility, although the beam of light which had changed its refrangibility was 
plainly discontinuous. When Indian yellow was used instead of lycopodium, the 
whole medium exhibited its sensibility when it was examined by the fourth method. 
In this case the suspended particles were so fine that the beam of light which had 
changed its refrangibility appeared to be continuous, though of course it was not really 
so. In observing with muddy fluids like these, it is almost necessary to employ 
absorbing media, since otherwise the effect of the light scattered at the surfaces of the 
prisms and large lens might lead the observer to conclusions altogether erroneous. 
171. The next test relates to the polarization of a falsely dispersed beam. Being 
engaged on one occasion in examining the effects of acids and alkalies on a weak 
solution of a sensitive substance, employing sunlight which had been merely reflected 
through a small lens, I met with a beam which had every appearance of having 
been only falsely dispersed, but on viewing it from above through a doubly refracting 
prism I was surprised at first by finding it unpolarized. It soon occurred to me that 
the beam must have been due, not to solid motes, but to excessively small bubbles 
of carbonic acid gas, the existence of which was thus revealed, though they were too 
small to be seen directly. The light being incident on these bubbles at an angle of 
about 45 °, which is very little less than the angle of total reflexion, the refleeted 
light would be almost perfectly unpolarized*. 
172. Water which had been merely boiled in a test tube gave a similar result. 
The unpolarized beam of falsely dispersed light was of course due in this case to the 
air which had been held in solution. This shows why long-continued boiling should 
be necessary, in order to free water from air. It is not that the affinity of water for 
air is so great as to be only gradually overcome, but that the air, immediately 
expelled from solution when the temperature rises sufficiently, is still retained in a 
state of mechanical mixture, forming excessively minute bubbles, the terminal velo- 
city of which is insensible. Accordingly it is not till larger bubbles are formed, by 
the casual meeting of a number of these small bubbles, that the air rises to the sur- 
face and escapes. 
173. With respect to the test of true dispersion depending on the change of 
refrangibility, it has been already remarked that in some cases the change is so 
slight, that if this test alone were applied, the observer might mistake true dispersion 
for false. However, it is only in rare cases that there is any danger of being deceived 
in this manner in the application of the test ; but on the other hand, in observing a 
muddy fluid or a translucent solid by the fourth method, the observer, if not on his 
guard, might easily be deceived by the effect of scattered light, and be led to mistake 
false dispersion for true. Thus suppose the medium to be water holding in suspen- 
sion particles of an insensible water colour, and the small lens to be placed a little 
beyond the commencement of the violet. Two beams of light would enter the lens, 
namely, a regularly refracted beam of violet, and a scattered beam of white light. 
* See note E. 
