330 ON THE CHANGE OF REFKANGIBILITY OF LIGHT.
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 independent 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 whim 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 <>f 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 // wore indeed seen with great, facility ; but I was not able to make out fixed lines beyond the end of t.he group l} that is, about the end of Knumhofer's map. However, the eyes of different individuals may differ much in their power of being affected by the highly refrangible rays. It must he confessed that, on looking in the direction of the prisms, a, good deal of blue light was seen, consisting of light, which had been scattered at the surfaces of the prisms and lens. This light., I hough fa,r from dazzling, was sufficient, to prevent the eye from seeing excessively faint objects, even though they might be well defined. For want of a heliosfat, I did not. attempt an experiment I was meditating for securing a more perfect isolation of the ultra-violet
rays*.
Sec unto B.