FIG. 54As was remarked above, this sodium line is
double, i. e., is really made up of two lines close
together. The distance between these two lines is
a convenient standard of measurement for our subsequent
work. This distance is so small that a
single prism scarcely shows that the line is double.
As we increase the number of prisms, the lines are
separated more and more widely. If, instead of a
prism, we use one of the best grating spectroscopes,
the two lines are separated so far that we might
count sixty or eighty lines between; and this fact
gives a fair idea of the resolving power of these
instruments. If we have two lines so close together
as to be separated by only one-hundredth
of the distance between these two sodium lines,
the best spectroscope will hardly be able to separate
them; i. e., its limit of resolution has been
reached.
The difference in the character of the lines from different substances is illustrated in Fig. 55. The spectrum that you have just seen is a photograph from a drawing, not a photograph from a spectrum. These are from spectra. On the right is a portion of the spectrum of iron, the other the corresponding portion of that of zinc. The enormous diversity in the appearance of the lines will be noted. Some are exceedingly fine—so fine that they are not visible at all; others are so broad that they cover ten or twenty times the distance between two sodium lines. This width of the lines depends somewhat upon the conditions under which the different substances are burned. If the incandescent vapor which sends out the lines is very dense, then the lines are very broad; if it is very
