POE KIECHHOEE’S SPECTEAL LINES. 
35 
numbers is necessarily (from the nature of the process) almost perfect. But between 
each of these principal lines and the next, the discordance rises gradually from 0 to a 
considerable value and then drops gradually to 0. Between E and G the value is large. 
The discordances, however, are systematic. There is no appearance of any saltus be- 
tween one principal line and the next ; but there is nothing to enable us to decide posi- 
tively whether there is a saltus at each principal line. 
It appears to me that we must look to one of the three following causes for ex- 
planation : — 
1. The actual inability of a formula of the fifth order to represent the wave-length 
in terms of the spectrum-measure with sufficient approximation. 
2. A change in Professor Kirchhoff’s method of observation at each successive prin- 
cipal line. 
3. A change in Ditscheiner’s and Angstrom’s methods at each successive principal 
line. 
The third of these suggestions is extremely improbable: and after remarking the 
harmonious flow in the course of the discordances in every interval between principal 
lines (a harmony which gives no small testimony to the care employed in both classes 
of measures, and to the accuracy of the interpolation-details in the computation which 
connects them), I am hardly inclined to advocate the second suggestion. I think it more 
probable that the real cause is to be found in the first. 
I remark, however, that this doubt affects only the physical question on the broad 
scale. It does not affect the possibility of computing with accuracy the wave-length for 
any one of Kirchhoff’s lines. By means of the comparison between the computed 
wave-lengths and Ditscheiner’s measures (at the end of the Tables) there is no difficulty 
in computing for any other line the correction that ought to be applied to the wave- 
length in the principal Tables, in order to exhibit the true wave-lengths on Ditscheiner’s 
scale, without appreciable error. 
I have now only to explain the Tables which follow. 
The principal Table, headed “ Conversion of Kirchhoff’s Spectral Measures into 
Wave-lengths in terms of the Millimetre,” corresponds, in extent, to Kirchhoff’s Tables 
in the Berlin Memoirs, 1861 and 1862. Instead, however, of adopting the broken 
arangement of these Memoirs, I have placed the numbers in a continuous series, and I 
have omitted the repetitions in Kirchhoff’s successive Tables. 
The Table consists throughout of four columns (one of which is occasionally blank). 
The first column Contains simply Kirchhoff’s measures ; and the second contains the 
corresponding wave-lengths, computed by the process described above. The third 
column contains Kirchhoff’s symbols expressing the darkness and the breadth of the 
lines ; 1 expresses a slight darkness, 2 a little darker, and so on up to 6 which is very 
dark ; a denotes a very narrow line, h a little broader, and so on up to g, which is a line 
of considerable breadth (the measure of the breadth is not given). The fourth column 
(where it is used) contains the contracted name of the metal which, by combustion, pro- 
