508 PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 
104. It has been already stated that the bands H were distinctly seen on common 
white paper, the substance usually employed as a screen in experiments on the 
spectrum, but that this was due to a change of refrangibility produced in the extreme 
violet rays. These same bands have been seen on paper in the experiments of 
others, though of course their visibility was not attributed to its true cause. By 
the method of observation described in Art. 100, or still better, by a method not yet 
explained, it may be seen that the change of refrangibility produced by white paper 
is by no means confined to the extreme violet rays, and those still more refrangible, 
but extends from about the middle of the spectrum to a good distance beyond the 
extreme violet. The distance to which the illumination can be traced by means of 
light merely scattered in the ordinary way, may be seen by examining the primitive 
spectrum. In the primitive spectrum formed on white paper and other white sub- 
stances, I have not been able to trace the illumination beyond the edge of the broad 
band H, which accords very well with the illuminating power of the extreme violet 
when received directly into the eye. 
Illuminating Power of the Rays of high.Refrangihility. 
105. The prolongation of the spectrum seen on turmeric paper was brought 
forward by Sir John Herschel as a proof of the visibility of the ultra-violet rays, or 
rather as a confirmation of other experiments which had led him to the same con- 
clusion. Of course, the experiment with turmeric must now be regarded as having 
no bearing on the question ; but from the way in which Sir John speaks of it, it 
would appear that he thought the other experiments not so conclusive as to be inde- 
pendent of the confirmation which they received from this. The experiment with 
the distorted spectrum, indeed, must now be put out of account, because in this 
experiment, as I have been informed by Sir John Herschel, the light was only 
thrown on a screen. Accordingly, the question of the visibility of these rays may be 
regarded as open to further investigation. 
While engaged in some of the experiments described in Art. 89, I had occasion to 
form a pure spectrum in air in a well-darkened room, the slit itself by which the 
sun’s rays entered being covered by a deep blue glass, so that no great quantity of 
light entered even at this quarter. Now, if ever, it would appear that the ultra- 
violet rays ought to be seen by receiving them directly into the eye ; for the blue glass 
was so transparent with regard to these rays that the fixed lines far beyond H were 
seen with facility, even on substances, such as white paper, which stand low in the 
scale of sensibility ; and the length of the spectrum from B to H was about an inch 
and a quarter, so that when the extreme violet rays entered the pupil, supposed to 
be held near the pure spectrum, not only the extreme red rays transmitted by the 
blue glass, but even the brighter part of the transmitted blue and violet rays fell 
altogether outside it. However, on holding the eye a few inches in front of the pure 
spectrum, so as to see the fixed lines distinctly, the bands H were indeed seen with 
