30 Refracted and Diffracted Spectra. |January, 
there is again a descent, which attains its lowest limit at 
3770, where « is 2°9100. This value remains constant to 
3820 of the scale, from which point there is a rapid ascent 
to the end of the series; but in this part of the table the 
values of « are somewhat uncertain. 
In some parts of the series, the differences in ascending 
or descending from term to term are pretty regular; while in 
other parts these differences vary more abruptly. Their 
general characteristic feature is an alternate rise and fall in 
their value, so that, were they represented graphically, they 
would exhibit a very wavy line, at intervals becoming 
straight. 
For this table, absolute accuracy is not claimed; but it is 
as nearly correct as it can be made in the present imperfect 
state of the observations. Future more accurate observa- 
tions may render it necessary to introduce into it slight 
modifications here and there; but it is far from probable 
that these will affect its main features; while it may even 
now be trusted in the calculation of the wave-lengths, which 
will be correct to the fourth place of figures in Angstrom’s 
millimetric scale. 
To illustrate the method of applying the scale to this 
purpose, let us take an example. Suppose we wish to 
ascertain the wave-length corresponding to the hydrogen 
line, which stands at 2796°7 of Kirchhoff’s scale. One 
advantage of taking the extreme lines of the spectrum as 
constants is, that it presents the general formula always in 
one shape, namely— 
i Y 
€e= 
Pd 
ZE NE 
where y is the wave-length to be found, and the other 
quantities are all given. 
In the case of the above hydrogen line, we have— 
p=2796'7— 404°5=2392°2 log. 3°3787975 
q= 3882'5— 2796°7=1085'8 ,, 3°0357498 
y constant 3478°0 5, 3°5413296 
The value of « corresponding to index 2800 is 3°2890 
and to index 2790 3°2790 
Difference 0’0100 
so that 3'2790+67=3'2857 is the tabular value of «, cor- 
responding to 2796°7 of index. 
. 
