286 ON THE CHANGE OF REFRANGTBILITY OF LIGHT.
i ment, except here and there, and they are not, I fear, quite so
| accurate as might be desired; still, I feel assured that no one
j viewing the actual object would feel any difficulty in identifying
1 the lines with those in my map, provided the circumstances under
which Ms spectrum was formed at all approached to those under ! which mine was seen when the arrangement as to focal length of
; the lens, &c. was that most convenient for general purposes.
The more conspicuous lines in the part of the spectrum represented in the map may conveniently be arranged in five groups, j which I will call the groups H, I, m, n, p. The group H consists
j chiefly of the well-known pair of bands of which the first contains
j Fraunhofer's line H; the second band I have marked k, in accord-
j ance with Professor Draper's map. The most conspicuous object
s in the next group consists of a broad dark band, I. This band is
1 between once arid 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 I Draper's map, while the first three lines of the group (if I do not
| mistake as to the identification) are represented, forming his
p group L. Under the circumstances to which the accompanying
f map corresponds, the band I appears as a very striking object,
!i perhaps, with the exception of the bands //, k, the most con-
spicuous in the whole spectrum. With a still lower power it ' appears as a very black and conspicuous line. A double line
beyond I completes the group I, 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 refrangible
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 conspicuous than its neighbours, but on account of its central situation. After a very faint group, | consisting apparently of four lines, comes another very conspicuous
I 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 I is a good deal broader than the second, but is not so broad as the
! band J?; the second is followed by 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