Wave-lengths in the Invisible Prismatic Spectrum. 207 



whose results, I hope, will supply useful data for those who 

 are interested in theories of dispersion. 



Table IV. — Approximate Errors in Wave-lengths by Briot's, 

 Cauchy's, and Redtenbacher's formulae for Cold Bands in 

 Infra-red. 



Comparison of Theories with Observation. 







Wave-lengths derived by extrapolation. 



n. 



Ob- 











served, 



From Briot's 



From Cauchy's 



From Eedtenbacher's 



By obs. 



X. 



formula. 



formula. 



formula. 



Value. 



Error. 



Value. 



Error. 



Value. 



Error. 



Value. 



Error. 



1-5714 



0-760 



0-760 



0-000 



0-760 



0-000 



0-760 



o-ooo 







1-5697 



0-815 



0-815 



o-ooo 



0-818 



0-003 



0-820 



0-005 







1-5687 



0-850 



0-850 



o-ooo 



0-853 



0-003 



0-862 



0-012 







1-56781 0-890 



0-891 



o-ooi 



0-900 



0-010 



0-915 



0-025 



2-230 



1-340 



1-5674 



0-910 



0-911 



o-ooi 



0-920 



0-010 



0-941 



0-031 



2-170 



1-260 



1-5668 



0-940 



0-942 



0-002 



0-960 



0-020 



0-990 



0050 



2-060 



1-120 



1-5636 



1-130 



1-170 



0040 



1-270 



0-140 



Imaginary. 



Imaginary. 



1-5616 



1-270 



1-336 



0-066 



1-730 



0-460 







1-5604 



1-360 



1-450 



0-090 



2-460 



1-100 







1-5576 



1-540 



1-750 



0-210 



Impossible. 







1-5572 



1-580 



1-800 



-0-220 









1-5544 



1-810 



2-105 



0-295 









to 



to 



to 



to 









1-5535 



1-870 



2-260 



0-390 









1-5520 



1-980 



2-460 



0-480 









1-5515 



2-030 



2-524 



0-494 









. Note. — 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 

 errors of X see Table II. "X observed" is either from a direct observation or 

 from an interpolation between two closely contiguous observations. 



It is evident that Briot's formula, though not exact, yet 

 gives results much more trustworthy than the others con- 

 sidered ; and it was employed in constructing provisional 

 maps of the normal spectrum from the prismatic, until an 

 apparatus was completed for determining the wave-lengths of 

 the invisible rays by direct measurement. 



We must evidently conclude from the numbers in Table IV. 

 and from the curve in Plate VIII., which embodies them, that 

 we in reality can scarcely assign any limit to the extent of the 

 infra-red prismatic spectrum ; and that far from the curve 

 having an asymptote parallel to the axis of X, as Cauchy's 

 theory requires, our curve (so far as we can follow it) rather 

 tends to ultimately coincide with a straight line cutting the 

 axis at a finite angle, and (if this axis pass through the point 

 n=l) at a great distance from the origin. 



