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