in the Invisible Prismatic Spectrum. 181 
Taste IV.—Appromimate errors in wavelengths by Briot’s, 
Cauchy's. and Redtenbacher’s Sormule, for cold bands in 
infra-red. ; 
(Comparison of theories with observation.) 
Wave-lengths derived by extra-polation. 
n Obsery’d| ————_____» _ 
From Briot’s From Cauchy’s la. 
osc ‘ Forticis, wovauie. From Redtenbacher’s Formu 
Value. Error. 
| 
Value. | Error. | Vaiue. | Error. | Value. |- Error. 
15714 | 0-760 | 0-760 | 0-000 | 0-760 | 0-000 | 0-760 | 0-000 
15697 | 0-815 | 0:815 | 0-000 | 0-818 | 0-003 | 0°820 | 0-005 
15687 | 0-850 | 0-850 | 0-090 | 0-353 | 0°003 | 0-862 | 0-012 
‘5678 | 0°890 | 0°891 | 0-001 0-900 | 0°010 | 0°915 | 0-025 | 2°230 | 1°340 
‘5674 | 0-910 | 0-911 | 0-001 | 0-920 | 0-010 | 0-941 | 0-031 | 2°170 | 1-260 
6668 | 0°940 | 0-942 | 0-002 | 0-960 | 0-020 | 0-990 | 0-050 | 2°060 | 1-120. 
6636 | 1130 | 1-170 | 0-040 | 1-270 | 0140 Imaginary. Imag inary. 
5616 1°270 | 1°336 | 0°066 | 1-730 | 0-460 
8604 1-360 | 1-450 | 0-090 | 2-460 | 1-100 
8516 1540 | 1°750 | 0210 | Impossible. 
15535 | 1:870 2°260 | 0°390 
15520 | 1-980 2°460 | 0°480 
15515 | 2-030 2°524 | 0°494 
oS Se! ee Taper | 
Nore—A part of the above values of n, where determined from observation 
by the bolometer, are liable to error in the fourth decimal place. For probable 
€rrors of 4, see Table II. “2 observed” is either from a direct — 
or from an interpolation between two closely contiguous observations. 
= evident that Briot’s formula, though not exact, yet gives 
results much more trustworthy than tbe others considered, and 
it was employed in constructing provisional maps of the 
+ 
2 
€ must evidently conclude from the numbers in table IV 
and from the curve in Plate.-V which embodies them, that we 
8reat distance from the origin. 
ith the danger of extra-polations presented to us In such — 
xamples as have been cited, we shall not attempt to generalize 
the results of our observations, further than to remark that, for 
rism in question, we find that the deviation tends within = 
the limits of observation to become proportional to the wave. 
