PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 471 
Once or twice a lens was tried which had a focal length about three times as great, 
but the light proved too faint for most purposes. In the third method it was some- 
times convenient to employ a lens of only 6|- inches focal length, but the 12-inch lens 
was employed in the fourth method, except on a few occasions, when the lens of 36 
inches focal length was used. With the 12-inch lens the length of the spectrum from 
the fixed line B to H was usually about an inch and a quarter. 
It will be convenient for the purposes of this paper to employ certain terms in a 
particular sense, but as some of these terms relate to phenomena which have not yet 
been described, it will be well previously to relate in detail what was observed in one 
remarkable instance of internal dispersion. 
Solution of Sulphate of Quinine. 
14. The effects of some pale coloured glasses in the case of this fluid have already 
been mentioned. But there is one glass of which the effect is still more striking. It 
is well known that a deep cobalt blue glass is highly transparent with regard to the 
chemical rays. Accordingly I found that a blue glass, so deep that only the brighter 
objects in a room could be seen through it, produced but very little effect when placed 
so as to intercept the light incident on the fluid. When placed immediately in front 
of the eye, at first everything disappeared except the light reflected from the con- 
vexities of the glass tube; but when the eye became a little accustomed to the dark- 
ness it was possible to make out the existence of the band. The contrast between 
the effects of this glass and of the pale brown glass already mentioned was most 
striking. 
15. When the fluid was examined by the second method, the dispersed light was 
found to consist of two beams, separated from each other at their entrance into the 
fluid, that is, at the vertical surface of separation of the fluid and the containing 
vessel, and afterwards still further separated by divergence. Of course each beam 
must have been made up of a series of cones having their axes diverging from the 
centre of the lens, and their vertices situated at its focus. The first beam, or that 
which was produced by light of less refrangibility, consisted of the brighter colours 
of the spectrum in their natural order. It had a discontinuous, sparkling appearance, 
and was plainly due merely to motes which were suspended in the fluid. On being 
viewed from above through a Nicol’s prism, it was found to consist chiefly of light 
polarized in the plane of reflexion. Taken as a whole, it served as a fiducial line to 
which to refer the position of the second beam, and thereby judge of the refrangibility 
of the rays by which it was produced. 
This second beam was a good deal the brighter of the two. Its colour was a beau- 
tiful sky-blue, which was nearly the same throughout, but just about its first border, 
that is, where it arose from the least refrangible of those rays which were capable of 
producing it, the colour was less pure. It had a perfectly continuous appearance. 
When viewed from above through a doubly refracting achromatic prism of quartz, 
MDCCCLII. 3 p 
