PROFESSOR STOKES ON THE CHANGE OF REFRANGIBILITY OF LIGHT. 477 
Professor Draper’s map. The most conspicuous object in the next group consists of 
a broad dark band, /. This band is between once and twice as broad as H, and is 
darker in the less refrangible half than in the other. With a lens of 3 feet focal 
length and a narrow slit it was resolved into lines, which is probably the reason why 
it is altogether omitted in Professor Draper’s map, while the first three lines of the 
group (if I do not mistake as to the identification) are represented, forming his 
group L. Under the circumstances to which the accompanying map corresponds, 
the band I appears as a very striking object, perhaps, with the exception of the bands 
H, k, the most conspicuous in the whole spectrum. With a still lower power it ap- 
pears as a very black and conspicuous line. A double line beyond I completes the 
group /, after which comes another remarkable group m, consisting of five lines or 
bands. Of these the first is rather shady, though sharply cut off on its more refran- 
gible side, but the others, and especially I think the second and third, are particularly 
dark and well-defined. I have marked the middle line m, not because it is more con- 
spicuous than its neighbours, but on account of its central situation. After a very 
faint group, consisting apparently of four lines, comes another very conspicuous 
group n, consisting of two pairs of dark bands followed by another pair of bands 
which are broad and very dark. The first of these is a good deal broader than the 
second, but is not so broad as the band H ; the second is followed bv a fine line. 
This is as far as it is easy to see ; but when the sunshine is clear, and the arrangements 
are made with a little care, a group of six lines is seen much further on. Of these, the 
first two are only moderately dark, and the first is rather diffuse ; they stand off a 
little from the others, and are a little closer together than the other four. Of the latter, 
the first, marked o, is very strong, considering the faintness of the light which it in- 
terrupts ; the second and third are faint, and difficult to see ; the fourth, marked />, is 
black like the first, and a good deal broader. The line p was situated, by measure- 
ment, as far beyond H as H beyond b. Once or twice in the height of summer, and 
under the most favourable circumstances, 1 have observed two broad dusky bands 
still further on. The first of these had the appearance of being resolvable into two. 
The excessively faint light seen beyond the second seemed to end rather abruptly at 
the distance represented by the border of the accompanying plate, as if there were 
there the edge of another dark band beyond which nothing could be seen. In order 
to see the dusky bands last mentioned, and even to see the group p to most advantage, 
it was necessary to allow the central part of the beam incident on the prisms to pass 
through them close to their edges, so that evidently a great deal of light was lost by 
passing by the prisms altogether. This circumstance, combined with others which 
I have observed, convinces me that the great obstacle to seeing the fixed lines in this 
part of the spectrum consists in the opacity of glass. Were glass as transparent with 
respect to the invisible rays of very high refrangibility as it is with respect to the 
rays belonging to the visible spectrum, I know not how much further I might have 
been able to see. 
