PROFESSOR STOKES ON THE CHANGE OF REFRANGIKILITY OF LIGHT. 467 
towards the light. On looking down from above, in a direction nearly parallel to the 
surface of the glass, a blue arc was well seen, extending only a very short distance 
into the fluid, and situated immediately behind the hole. As this arc, though 
extremely distinct, was not of course what could be called brilliant, I did not at first 
venture, for the experiment I had in view, to use any but pale glasses. Having no 
direct means of determining which were opake with regard to the invisible rays 
situated beyond the extreme violet, I sought among a collection of orange, yellow, 
and brown glasses, which, from transmitting mainly the less refrangible rays, seemed 
the most likely to absorb the chemical rays. I presently found a pale smoke-coloured 
glass, which, when placed immediately in front of the hole, prevented the formation of 
the blue arc, although when placed immediately in front of the eye it transmitted a 
large proportion of the light of which the arc consisted. The colour of the arc was 
of course modified, and rendered more nearly white. 
On trying other pale glasses, I found one of a puce colour, which, when placed in 
front of the hole, allowed the arc to be formed, though it absorbed it when placed in 
front of the eye. A yellow, and likewise a yellowish green glass allowed the arc to 
be seen in both positions ; but its colour was decidedly different according as the 
glass was placed in front of the hole or in front of the eye. The breadth, too, of the 
arc was differently affected by different coloured glasses placed in front of the hole, 
some causing the light to be more, and others less concentrated towards the surface 
of the test tube than when the incident light was unimpeded. 
8. The sun’s light was next reflected horizontally into a darkened room, and allowed 
to pass through a hole in a vertical board which was placed in the window. The 
hole contained a lens of rather short focus. On placing a test tube containing the 
solution, in a vertical position, in front of the lens, at such a distance that the focus 
lay some way inside the fluid, the narrow blue band described by Sir John Herschel 
and the blue beam mentioned by Sir David Brewster were seen independently of 
each other. On trying different coloured glasses, which were placed, first in front of 
the fluid, and then in front of the eye, it was found that the blue beam, as had 
previously proved to be the case with the narrow band, was for the most part dif- 
ferently affected according as the glass was placed so as to intercept the incident or 
the dispersed light. Moreover, the long blue beam and the narrow band did not 
behave in the same manner under the action of the same coloured glass. 
9. To my own mind these experiments were conclusive as to the fact of a change 
of refrangibility. Admitting that the effect of a coloured glass is simply to stop a 
certain fraction of the incident light, that fraction being a function of the refrangi- 
bility, it is plain that the results can be explained in no other way. It must be 
confessed however that these results are merely an extension of that which precisely 
constitutes the peculiarity of the phenomenon. For, take the case of the narrow blue 
band formed by ordinary daylight. Imagine a glass vessel with parallel sides to be 
filled with a portion of the solution, and placed so as to intercept, first the incident, 
