PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 559 
or else to view it in small thickness, since otherwise the whole of the region in which the bands 
occur is absorbed. The bands are five in number, and are equidistant, or at least very nearly so. 
The first is situated at about three-fifths of a band-interval above D ; the last coincides with F, or, 
if anything, falls a little short of it. The second and third are the most intense of the set. I have 
carefully examined the solution for change of refrangibility, and have not found the least trace. 
Ferrate of potassa shows nothing remarkable. 
By means of the bands just mentioned, the colour of permanganate of potassa may be instantly 
and infallibly distinguished from that of certain other red solutions of manganese, the colour of which 
some chemists have been disposed to attribute to permanganic acid (see a paper by Mr. Pearsall 
‘On red Solutions of Manganese,^ Journal of the Royal Institution, New Series, No. IV. p. 49). 
Note E. Art. 17 I. 
If we suppose the angle of incidence exactly equal to 45°, assume ^ for the refractive index of 
the fluid, and apply Fresnel’s formulae to calculate the ratio of the intensity of light reflected 
at the exterior surface of a bubble, and polarized in a plane perpendicular to the plane of inci- 
dence, to that of light similarly reflected and polarized in that plane, we find 0’228 to 1, a ratio 
which certainly differs much from one of equality. But in order to render the two intensities equal, 
it is sufficient to increase the angle of incidence by only 3° 35' ; and in fact, as a matter of con- 
venience, the position of the observer was usually such that the deviation of the light was somewhat 
greater than 90°, and therefore the angle of incidence somewhat greater than 45°. 
Note F. Art. 191. 
I have since received a slab of glass of the kind here recommended, which has been executed for 
me by Mr. Darker of Lambeth, and which answers its purpose admirably, the medium being emi- 
nently sensitive. Besides its general use as a screen, this slab, from its size and form, has enabled 
me to trace further than I had hitherto done (Arts. *J5, 76) the connexion between certain fluctua- 
tions of transparency which the medium exhibits and corresponding fluctuations of sensibility. 
Note G. Art. 192. 
Paper washed with a mere infusion of the bark of the horse-chestnut is quickly discoloured ; but 
a piece washed with a solution which had been purified by chemical means remained white, and 
proved exceedingly sensitive. 
Note H. Ajrt. 204. 
I have since ordered a complete train of quartz, of which a considerable portion, comprising among 
other things two very fine prisms, has been already executed for me by Mr. Darker. With these 
I have seen the fixed lines to a distance beyond H more than double that oi p’, so that the length 
of the spectrum, reckoned from H, was more than double the length of the part previously known 
from photographic impressions. The light was reflected by the metallic speculum of a Silber- 
mann’s heliostat, which I have received from M. DuBOSca-SoLEiL. With the glass train the 
group p was faint, but v\uth the quartz train there was abundance of light to see not only the groups?, 
but the fixed lines as far as H^l, or thereabouts. From the group n to about the middle of the new 
region, the lines are less bold and striking than in the region of the groups H, I, m, n, but the latter 
MDCCCLJI. 4 C 
