1874.] Refracted and Diffracted Spectra. a3 
The mode of finding the value of «, when both the position 
of the line in the scale of the spectroscope and the wave- 
length are given, and held to be absolutely exact, differs but 
little from the foregoing. A certain value of « must be 
assumed, and when log. yé and its log. are found as in 
the preceding calculation, the log. of the log. of y, as given 
by observation, is to be subtracted from it. If the value 
of « has been accurately assumed, then the difference should 
be exactly equal to log. «. If the difference be greater than 
this last, then the assumed value of « must be increased ; 
if the difference be less, then the value of e€ must be 
diminished. A little experience will show the extent of the 
requisite increase or diminution, which varies in different 
regions of the spectrum. 
Thus, in the foregoing case, if from the log. 
2 NESE Gui aie ler ener ean, Semen 
We subtract log. of log. of the relative wave- 
length corresponding to 4340°1 of the milli- 
MCESC HC oe Se cere a ping hs, oat, Sie ee ee OO OLOTOLE 
UMMC Me? Sse ays Aye vias on alo 0°5166222 
Myhich is less than log.'c by w  ., 2... ..., 00000057 
OD Ee ena tee Op LOO2 ZO 
Thus showing that the value of e¢ should be slightly 
diminished. The requisite diminution is 00015, making the 
exact value of e, on the assumption that the observations 
are absolutely correct, 3°2842. 
It will hence be perceived how very sensitive is the expo- 
nent, and how considerable a variation it may undergo even 
with a scarcely appreciable difference of wave-length, or in 
the position of the line in the spectroscopic scale. 
In the table (p. 31), the columns marked K contain the 
degrees in Kirchhoff’s scale; while those marked e contain 
the corresponding values of the: variable exponent. The 
table is submitted in the hope that, notwithstanding any 
slight imperfections it may be found to contain, it may prove 
useful to those who are engaged in spectroscopic investiga- 
tions. 
0°5348133 
VOL. IV. (N.S.) F 
