1872.] its Imperfections and their Remedy. 53 
two spectra, each division of M. Kirchhoff’s scale may be 
reckoned equivalent to a division and a half of M. Ang- 
strom’s. It is only necessary, therefore, to add a half more 
to each of Kirchhoff’s intervals to make them equivalent to 
Angstrom’s. The following table exhibits the intervals 
between the lines in the two spectra thus compared :— 
Angstrém’s. Kirchhoff’s. Differences + Differences — 
A—B. 737 285 452 
B—C. 305 153 152 
C—D. 667 463 204. 
D—E. 627 781 154 
E—F. 408 834 426 
F—G. 553 1162 609 
G—H. 374 1542 1168 
3671 5220 2357 808 
A simple inspection of this table suffices to show how 
much the lines are displaced from their true relative posi- 
tions by the action of the prisms in M. Kirchhoff’s spec- 
troscope. 
For the sake of further comparison, suppose a spectroscope 
to be constructed with prisms having the same refracting 
angles as those in M. Kirchhoff’s instrument, namely, three 
with the angle 45°, and one with the angle 60°, but composed 
of flint glass, No. 30 of Fratinhofer. The following are the 
ultimate differences of deviation in seconds between the 
different lines, which would be given by such an instrument: 
eee). =f ES FG: G—H. 
1826, 4902, 6524, 5926, 11642, 10458. Sum 41,278 
For a short interval like that between the more and less 
refrangible of the two lines marked D, the differences of the 
indices of refraction may be assumed to bear to the differences 
of wave-length the same relation as they do in the case of 
the adjacent principal lines, which are here the less re- 
frangible D and the more refrangible E. The difference 
between the relative wave-lengths of these two lines is 
o°118g9003, and the difference between the wave-lengths of 
the more and the less refrangible D is o°0011378—the former 
difference being nearly 104°5 times that of the latter. The 
difference between the refractive indices of the less re- 
frangible D and the more refrangible E for flint glass, No. 30, 
is 0°006768 ; and on the supposition that this difference is 
104°5 times greater than that between the indices of the 
more and the less refrangible D, the latter difference will be 
